##RAW: So long as they know the target is within the cube, then it can be done.

Beyond just the RAW: (Not so much RAI in-and-of itself, but you can see it in the same vein.)

However, let's say they were following a character who took levels in Rogue and has Cunning Action. Presuming you were treating things as though the characters were in Combat, regardless of if it was expressly stated or not, you could say it went like this:

1. Target walks down street.
2. PC is following on their "turn".
3. Target casts Invisibility on himself.
4. Target uses Cunning Action to take a Dash action to get away.
5. PC catches up to the area he last saw the Target in and casts Slow in a given area.
6. Target is outside the Area of Effect and is therefore unaffected by Slow.
7. PC wouldn't know if the spell worked or not and would have to decide how to proceed with this uncertainty. Depending on if the Target notices or not, a formal Combat order could begin.

This is just an example for showing the other side of the coin. In this case, the spell fails. In your scenario, the spell succeeds. If it was a matter of Slow specifying "that you can see", the Slow spell would then fail, but it doesn't. Essentially, all the Invisibility did was add another layer of potential failure to the Slow spell. Instead of the target simply having to succeed a Wisdom ST, now the caster has to be sure they are including their target in the area of effect or else it fails, and the PC won't know whether it failed because they missed or if their target succeeded the roll, which makes continuing their actions all the more difficult.

RAW: So long as they know the target is within the cube, then it can be done.

Beyond just the RAW: (Not so much RAI in-and-of itself, but you can see it in the same vein.)

However, let's say they were following a character who took levels in Rogue and has Cunning Action. Presuming you were treating things as though the characters were in Combat, regardless of if it was expressly stated or not, you could say it went like this:

1. Target walks down street.
2. PC is following on their "turn".
3. Target casts Invisibility on himself.
4. Target uses Cunning Action to take a Dash action to get away.
5. PC catches up to the area he last saw the Target in and casts Slow in a given area.
6. Target is outside the Area of Effect and is therefore unaffected by Slow.
7. PC wouldn't know if the spell worked or not and would have to decide how to proceed with this uncertainty. Depending on if the Target notices or not, a formal Combat order could begin.

This is just an example for showing the other side of the coin. In this case, the spell fails. In your scenario, the spell succeeds. If it was a matter of Slow specifying "that you can see", the Slow spell would then fail, but it doesn't. Essentially, all the Invisibility did was add another layer of potential failure to the Slow spell. Instead of the target simply having to succeed a Wisdom ST, now the caster has to be sure they are including their target in the area of effect or else it fails, and the PC won't know whether it failed because they missed or if their target succeeded the roll, which makes continuing their actions all the more difficult.