Need Some Help

I was doing some routine work last night on the tank and checking out all the fish to make sure everyone is getting along well. Checked out my Harlequin shrimp, he has two homes. One over in the right back corner of the tank, and when he's eating one under a rock cave right up front, super cool to watch. My question is that while looking at him last night I noticed a black thing hanging out on the rock above him. It's a pretty shinny black color with 2 antenna like things coming from the front that have a white stripe on each one. it's probably about 2 1/2 maybe 3 inches long and I've never noticed it before. Probably a couple inches wide and it seemed to shrink and get bigger at different times. I think it may be some sort of sea slug, but of the one's I've looked at online nothing really seems to fit the bill. The tank has been running for a almost 10 months now and I haven't added any new rock in about 3..just coral. So no clue if it's a hitchhiker or not. I tried to get a picture but it wasn't working well on my phone so sorry for that. Any help from anyone would be greatly appreciated. It doesn't seem to be bothering anything from what I can tell, but not sure.