what is this?

definetly looks like a nudibranch, they have very specilized diets normally, so if it was between meals it might not appear to eat much. I would not trust it unless you know exactly what it is.

Any new monti's in the tank?

If you have any new monti's in the tank and that little .25 inch nudi is white, I may have some really bad news for you.

I looks like a monti eating nudi, but the pic is small and I cant tell. You will know soon enough if it is a monti eater. Your monti's will tell you.

To me it looks like a nudi that eats leathers. Did you buy some soft coral or leather?