ID Please

[manfred]

Hi all,
Name is Fred, live at Regina Beach. New to the reef hobby.
Set up tank about a month ago. Started with live sand, live rock (from Bayside Corals, Saskatoon) and most recently the clean-up crew. I was shocked to find what I think is a really big hitch-hiker on the live rock, it must have been hiding for all these weeks in my tank. It decided to make it's debut today and has been slithering around the tank at a fast pace. I would really appreciate if anyone could ID it. My concerns are: will it be harmful to corals, anemones or any future inhabitants in the tank? If safe to keep, does it need to be fed?
Thanks.