I think the question you should be asking is not 'what is this?' but rather 'should I keep this in my tank?'
I had a bristleworm in my tank that grew from 1" to over 6" in length in only a few months. I did some research on it and got mixed reviews, with some people saying that bristleworms were ok and others saying they are not ok. Finally I asked the lfs about it and they said 'why take the chance?'
That sounded like good advice to me so I captured and flushed the bristleworm down the toilet.
I think that's probably good advice for that orange slug thing. At best it is a harmless detritus eater but at worst it might be harmful to fish or coral. Why take the chance?