Thanks for the reply scubamarli.
Why do you think it may be an isopod? It certainly made me wonder, but I think it's a lot more shrimp-like than isopod (not that I know a lot about isopods, except the little sponge isopods I film). When reading about isopods it states that "Isopods lack an obvious carapace, which is reduced to a "cephalic shield" covering only the head", whereas these seem to have a well defined carapace. I think my wife has some clearer headshots and you can see a small rostrum.
It's been surprisingly difficult to ID them. When you start to look for them you can find them pretty easily, so I don't think they are very uncommon, but they are usually hidden on the underside of the Tubastraea micrantha, and so are probably usually overlooked by guides and divers.