Last Sunday we added in a female marble molly to our tank to try and improve the ratio of male to female molly. Previously, we had one male and one female pair that we inherited with the tank.
For the first few days, our male couldn't leave the new female alone and was constantly trying to mate with her.
However, on Saturday we noticed some aggressive behaviour. He'd display his anal fins in a downward 'V' and then charge at her. Once she swam away he gave up but would start again later.
He isn't being aggressive to any other fish in the tank, including our other female molly. I'm also sure it's not mating as I've seen him mate before and it's completely different.