That doesn't look like a normal amphipod. I have those too. It is not a mysid either. It does look more towards isopod... That's what I wanted to confirm. http://reefkeeping.com/issues/2006-02/bp/index.php
Isopod for sure, though from the picture I couldn't say what species. Not all of them are parasitic, if you notice any in or around the gills of your fish, that would be the time to worry.