Notice the one eye spot, that is a species of Copepod (small crustacean), harmless detrivore/herbivore. You'll probably see hundreds if this is a new tank.
So do isopods. The outline is fuzzy but I suspect it's a sphaeromatid and the dark areas are something subdermal rather than eyes. The placement of the legs & the dorso-ventral flattening of the body are typical of isopods.
