sudden slug population

my first real score on some very cool fuji rock came with all kinds of suprises. feather dusters, clams, and one shelled creature that looked like a small abalone to me. the next morning however i noticed it parked on a clam. then i noticed all the rest of the clams were gone except for their shells. he's gone now. so when i noticed these slugs yesterday i got worried that another pest has reared its ugly head. I haven't introduced any new rock lately so i dont know why they have suddenly appeared. i'm having trouble getting pics so I'm hoping i can describe them and get some opinions. They are grey slug like critters about 1/2 inch long and appear to have dark spots. They have antenae and move incredibly fast. they bury themselves part way in the sand at times. i dont see them leaving clean spots so i dont think they are algae eaters. The first one i saw was running down the back of the tank after i turned on the lights. maybe they like the night life. Any ideas? I'll try to get some pics as soon as i can catch one in the right spot.

 

It sounds like a Stomatella. Google it and see if it looks like what you have. They are harmless and actually a good member of you cuc.