Sea Slug? ID Please

[Frigideus]

Normal 4 am tank watching, doing a little scrubbing to fight off the uglies. Noticed this thing. I can't find anything that looks like it on the web. Pretty plain looking and also decently big. About 1 inch long.

I hope I don't need to get rid of it since it's full of personality but please tell me if it's safe to keep around or not!

 

[Crabs McJones]

Nudibranch of some sort. Not sure the exact type