Yep Fusilier Damsels (Lepidozygus tapeinosoma) stunning fish once it colours up, gets to about 4". Not aggressive and eats any normal aquarium fare. Does well in groups
It could be a couple of reasons. Mine took about 2 weeks before they settled enough to start to colour up or you say only have one, that may cause it stress, they are a group fish, do you have any other small open water swimmers, like anthias for it to swim with.
Did you LFS have any more? if so I would recommend you get a couple more, watching them swim as a group is a joy.