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If he is bullying a female it typically means she is ready to produce fry. Some male mollys exhibit these types of behaviors. It seems Dalmatian Mollys have a strong tendency to bullying.
I had 3 mollys, they were fine together, 2 died and the other became a right little bugger. I ended up have to get rid of it. I think if they're kept in groups they are ok - well that's my (limited) experience.