Critter ID please

[JBAqua]

I am new to CanReef and new to s/w and have a question. My tank hs just been set up for 1 month. I spotted this unknown critter in my tank. (I think it must have come with my live rock).

Someone mentioned to me that it might be some kind of nudibranch. Is it? If so, what kind is ir and is it a problem or just a neat creature to have in my tank?

Sorry for the picture quality (I'm no good at closeups of tiny creautures . The creature is black and just under 1/2 inch long.



I'd appreciate any insight you can provide. Thanks.