looks to me like it could be a Dirona albolineata, a snail predator, also eats bryozoans, ascidians and sea anemones.
but it is hard to tell in the picture, do the tendrils on the back have white highlights?
Placida dendritica?
(how big is it?)
edit: nevermind, you said a little less than an inch, I think that may be too big for these guys, but google it and see if it looks simmilar to you too
