I agree, most likely a softy eating nudibranch. It will look almost exactly like the coral itself. Also, have a look with a flashlight at night, they're a little more daring under cover of darkness.
The white one below is similar to the one I had (I thought my colt was reproducing, then one night I found it moving along the front glass).
Taken from the medslugs.de website.
Tritoniopsis elegans/frydis
