+1 on amphipod I had some come on the live rock I bought, they grow in population quickly which is great! You will notice them die off and be scarce then suddenly theres a ton again depending on your stocking. They self regulate!
Should I be worried about these guys? They are nocturnal, and seem to make nests on or around all of my corals in particular. They have't seemed to bother the fish or shrimp. The grids on the graph paper are 1/4 in for reference.