A fellow reefkeeper took this picture: It is a Cirolanid Isopod, a very tiny creature that feeds on fish similarly to ticks on dogs. Usually these appear affixed to the body of a fish, or gills. Do some research on how to remove them - they are unwelcome pests. If left unchecked in your tank, fish will die. You will need to leave your aquarium fish-less for 2 to 4 months to erradicate these. NOT Reef-safe.