A little tough to make anything out from that picture, but since you have a pair I'd venture to say they're a member of the Family: Trapeziidae and very possibly of the Genus: Tetralia since they're hosting in an Acropora spp. They are harmless herbivores.
The crabs are more than likely porcelain crabs (harmless).
THe acro in the pic look like a valaida sp. sort of creamish white all over with purple blue tips that look sort of tri color oil slickish right?
Crabs are white body and claws with a dark stripe across their forehead. Acropora is light blue with darker tips. It is under 250W of HQI 10,000K illumination in a 30g Bowfront. It has really gained color since introduction. Its only neighbors are a Coral banded shrimp and 2 4" Tridacna Crocea clams. All are happy. I am really happy with the mix of color & wildlife.
