Well I am looking for a positive ID.
This came in on some live rock a while back. When I first found him he was a small baby. I have not removed him. I have been keeping a very close eye on him and found that he does not go near any coral and strictly keeps on the hunt for algae. Also this is not nocturnal. Only comes out when the lights are on.
The picture is not the best so I will continue to try and get a better picture.
Thank you
Mark

http://photo.reefkeeperz.com/users/3mm3/Mystery%20hitch%20hiker.jpg

http://photo.reefkeeperz.com/users/3mm3/hh.jpg

I can't offer a specific ID on him. But any nudi or slug I can't ID I isolate until I get that postive ID. If I don't get it, the critter doesn't go back in the tank. Visually not catching him at "bad habits" doesn't guarantee the critter is safe.

If nobody speaks up here go to project DIBS (the link is to the left of the page) and ask Ron Shimek.

Could be a lettuce nudibranch... they are quite hard to pin down though because the same genus can look amazingly different depending on where it is from..