Need Identification Please

[BTA Love]

I seen this in my tank last night, took a couple pictures with my phone, and this morning it is gone. I was up late trying to look it up online and again this morning but I am just not finding it. Thank you so much.

 

[ahiggins]

sorta looks like the underside of a cowrie or a chiton.
Try looking up those two to see if it looks familiar.
Its stuck to the glass, not the rock-correct?

 

[BTA Love]

No, lol. It's in the rock. And now that the lights have come on, it is back in the hole that was empty an hour ago. I'm thinking some type of sea slug but am wondering what kind and if it is bad or good.