It looks like a fish lice I had a few of those in my goldfish tank before I just tweezered them and added aquarium salt and never saw them again. I kept them in a cup for kicks then finally finished it off for boiling water.
its a fish lice. i see them usually sticking on fishes like kois and goldfishes. it crawls fast. sometimes it's hard to tell because it blends well with the scales. it must be destroyed or your fish will suffer.