I can't see his/her anal fin, as there's another fish photobombing...
Here's some information that might help you.
"Molly (poecilia latipinna) – this type of livebearers need more observation than the other livebearers. The common male Mollies have a colorful tail (like bluish or greenish color) having a larger dorsal fin, and is usually thinner (viewed from top) than the females. Generally, the best observations are focused on their “gonopodium” or what we called the modified anal fin. It is a tube-like fin at the lower backside next to the belly of the fish."