Mayfly nymphs (or similar- have a google of them to see if it looks familiar). They often come in on plants. If you've got big fish, they will just eat them. Some larger nymphs will eat small fish like neons or endlers, but these look like mayfly nymphs because of the three tails so they shouldn't be a problem for any fish.
They shouldn't do, I think mayfly nymphs normally eat crap detritus and whatnot. It's dragonfly nymphs you really have to worry about. If you're concerned then just scoop them out, or squish them and the fish will eat them then.