my son has a sail fin molly and he says it looks like it is foaming at the mouth what could be causing this

is the foam black or white? i have seen many growths like this on other fish mostly tetras, it could be a fungal infection called velvet, it could also be a mouth infection had it been bit or in a fight with another fish and wounded. you may need to treat with some meliflex to get rid of the disease or infection, if it's a parasite, meliflex will help with that as well.
hope this helps
Brandon