You need to read up on what mattes are and ho they are used. If you already have cut out your object you don't need an extra mask, anyway. It's simply redundant and does nothing that you couldn't do with an inverted matte.
Ae does not get editable Ps layer masks (that are made of pixels). when you mask a layer in Ps, whether you merged the layer with the mask or not, Ae will get the layered merged with the alpha, and cropped to the mask boundaries (as long as you imported - retain layer sizes). if you change the mask in Ps, the alpha information in Ae will change too. on the other hand, you can create a vector mask in Ps and get editable mask in Ae (if you choose - editable layer styles on import). but this is mainly used for Ps shape layers and not actually a mask for still image. photoshop's masking is way more accurate then Ae and you should prepare your still images in Ps before you import them to Ae. so the mask information is not that useful for you anyway. there are cases where you want the mask in Ae and you can create in Ps a separate layer with just the alpha (solid with the same mask applied) and use that as an Alpha matte (Ae's version to a clipping mask - sort of)
more information here:
Thanks so much, sometimes you just need to know what to look for