That appears to be one of the copepod eating flatworms. Not harmful and won't accumulate in the tank. The ones that become problematic are the brown ones with orange/red spots on them. Those are partially photosynthetic (the kind above is a hunter and not reliant on light).
I started out with one and now every time I baster I have dozens. I thought the first one was cute and kept the fish from eating it until it hit the substrate. I am not sure if I regret this now. Maybe my six-line will eat them...
