Your DM will need to adjudicate.
The argument FOR allowing
Rules-as-written, there is a strong argument for this.
In the Basic Rules, under Unseen Attackers and Targets, it says:
When a creature can't see you, you have advantage on attack rolls against it. If you are hidden--both unseen and unheard--when you make an attack, you give away your location when the attack hits or misses.
And the Manifest Mind feature says:
Whenever you cast a wizard spell on your turn, you can cast it as if you were in the spectral mind’s space, instead of your own, using its senses.
So, if you are attacking, the rules say you should get advantage.
Of course, once you attack, you're no longer hidden.
The argument AGAINST allowing
However, the target has no way of knowing that it was you and not the Manifest Mind that cast the spell. From the target's point of view, the floating thing attacking it wasn't hidden at all.
The rules for Unseen Attackers and Targets says "when you make an attack, you give away your location when the attack hits or misses." Clearly the narrative is that the attacker doesn't see you, sees the attack coming from your space, then knows where you are. If the attack appears to be from the Manifest Mind, it is not much of a stretch to say the target sees where the attack is coming from (or where it appears to be coming from), so no advantage.
You win either way
If the DM rules that you do get advantage, then you get advantage, so that's a win.
On the other hand, if the DM rules you don't get advantage, that's a win, too. That means you remain hidden, and your target does not get to know where you are, at least not because of your attack. For a squishy wizard, staying hidden is an advantage, even if you don't get advantage.
How to adjudicate
Either way it's a good trick. The DM is probably not risking dangerous precedent either way and may have a preference one way or another. A discussion with the player is probably warranted; which way is most fun? Both seem like good tricks, situationally useful, and not dramatically overpowered. As a DM and a player I would lean toward no advantage, because I think it makes the most narrative sense and I think the idea of the wizard remaining hidden is fun.