Yup, amphipod. (More than likely of the Gammaridae family)
Some consider them good guys, as they eat detritus and micro algae.
Others with no fish to eat them consider them a nuisance. In large quantities, they can annoy corals when they crawl all over them.
As long as you have some form of a predator (damsel, wrasse, hawkfish, etc.) they will keep numbers in check and supply a nice little snack for the fish.