my eyesight isn't great and my screen is small, but it kind of looks like an isopod/amphipod. if so, it would be a parasitic one.
can you try to gently take it off with fine tweezers? if not, if you confirm that it's a parasite, look for a post by uberfugu about a parasite INSIDE the gill of his anglerfish. he finally got it out with some sort of treatment, but I don't remember exactly what he did...