With the aptasia information in mind...I would say peppermint shrimp
.
Possibly also a Catalina peppermint shrimp (
Lysmata californica), but you'll want to do more comparisons with pics online yourself.
Those guys wont eat aptasia, and are unsuited to the higher temperatures of the average tropical reef tank, they don't usually live long. Really should be kept at 15*C-18*C.
How big is it? If it's quite small it could still be a peppermint, the reason I believe it could be
Lysmata californica is it looks slightly more stout than all the peppermint shrimp I've seen, and also looks kinda big. Could just be the photo though,
Also, where are you located? If your in the UK you can be fairly sure it's a proper peppermint.