To start with, I'll point out that this scenario is extremely similar to a mage having readied a spell, and then releasing it on the next turn. In both cases, the casting occurs during the first turn, and the effect occurs during the second.
For anyone who considers these scenarios identical, then one should just apply the same logic used for readied spells. And this is answered here: Can a spell be readied to trigger when its intended target comes into view?. That is, the target of the spell is not even determined during casting at all. Thus, there's no such thing as "a target that moves out of sight". The target's not a target until the second turn, and the spell works completely normally, regardless of whatever movement might have occurred by any potential targets prior to release of the spell's effect.
This interpretation is, as noted in that answer, loosely supported by this reply on Sage Advice:
For readying a spell or other action, does the target have to be in range? Your target must be within range when you take a readied action, not when you first ready it.
Note that the conclusion derived from that answer, for the other RPG.SE answer, is not directly stated. It's inferred as a natural consequence of that more general rule. But it does seem reasonable.
That said, it is just an inference and the rules don't provide exact guidance. So, assuming one does not consider the effect of Slow to be identical to a readied spell, or if one does not agree with the RPG.SE answer regarding readied spells and targeting, then we have to go further in our analysis. Read on… :)
Can William complete his spell that targets Wanda, who is now out of sight?
Yes, no problem there. They cast the spell using their first action and nothing changes after that that would prevent the spell from being completed. But, the effect of the spell fails to hit Wanda. This introduces the question as to what that effect actually is.
As I see it, there are two options:
- The bubble of acid follows the target
- The bubble of acid lands where originally intended
The latter option is simpler to use, but the former is supportable because, well…it's magic.
From the spell description:
You hurl a bubble of acid
You can cast the spell, no problem. That happens right away. But being slowed, the effect of the spell is delayed. And the effect of the spell is that "You hurl a bubble of acid".
If one has ruled that the bubble follows the target, then by the time you actually do that (using your action in your next turn), the target has obtained full cover. You hurl the bubble of acid toward the target, but they are protected by the cover, which is what your bubble of acid winds up hitting.
Depending on what the cover is, that may or may not have any further interesting effect (a stone wall might get slightly marred, but would otherwise be fine).
If one has ruled that the bubble lands where originally targeted, then it still lands there. If the target has moved behind cover by that time, they are protected. (I'd note however that if they just move 5', still within the range of a two-targeted splash, then if the cover is only between them and the caster, rather than between them and the originally targeted location, they'd still take damage under this interpretation.)
Nicely, the rules as written also fit the narrative. You're slow. It is not surprising that your targets may be able to take advantage of that slowness, by ducking, hiding, or otherwise avoiding your attacks.
It's not quite "bullet time", but it's the same basic idea. Your targets' reflexes have essentially been enhanced, relative to your own.
What happens when there are two targets?
Under one interpretation above, the direction of the cast bubble should change as the targets move. If there are two targets, then what? If neither target moved, then the bubble would land between them, splashing them both. It's thus reasonable to rule that should either move away from the other, the bubble will land between them, potentially splashing neither depending on where they moved (i.e more than 5' away from each other).
Under the other interpretation, the movement of the target or targets is irrelevant (and so it's easier to decide what happens).
What happens if the target (or targets) moves, but not into cover?
I think the answer to this can be inferred from all of the above. But it's worth noting that the direction of the bubble may or may not change after casting, depending on how one rules (either is valid, according to what's written). If one rules that it does, then if there's only one target, they will get hit if they don't achieve cover with their movement. If there are two targets, neither might be hit, depending on whether the location between them is too far from either to have any effect.
However, taking the main point above with the two-target scenario, we can see that if one is ruling that the direction of the bubble is not changed after the spell is cast, then if one or the other of two targets move before the effect occurs, the bubble might still hit one or the other of the targets, depending on whether they have moved far enough from the originally targeted location (since that's where the bubble will land, under that interpretation). Likewise a single target would be hit only if they remain close enough to the original location. They could avoid the bubble entirely even without achieving cover.
To sum up: the rules don't provide explicit guidance about whether the direction of the acid bubble changes after casting, so there will have to be a ruling determining that. Once that has been decided, it's possible to completely understand how the spell works under the effect of Slow.