I think what is happening is that your white butterfly skirt tetra's are biting the fins of Molly. This is because the white butterfly skirt tetra are a semi-aggressive fish. They bite the fins of other fish. Now, that could to any non-Attenborough fanatic seem like nothing important. BUT, I am a Attenborough fanatic. In blue planet there is a scene where a cleaner wrasse takes a bite out of a Manta ray and her partner chases her around their coral home. And then David Attenborough says "This is the fish equivalent of a stern telling off". So I THINK your white butterfly skirt tetra are biting the fins of molly and he is chasing them as a way of saying "please stop or don't do that".