I had this issue with my mollies too as they started to get older. I am assuming that you are feeding flake foods, if you are, try feeding frozen and fresh. Before my old molly died and I was feeding them nothing other than flakes, she had this issue too. Anyway, she had babies before she died and I have since started to feed them frozen (about $6.00 a pack at the lfs) and fresh veggies. Since then, I haven't seen anything of this sort in this litter of mollies as they have started to age as well.
The fresh veggies they eat is not limited really. I feed mine, zucchini, green squash and avocado (that is their FAVORITE). They will also eat fruits. I buy a batch and will cut it up and freeze it. It holds for a few months in the freezer. Take a few peices out 3 times a week, put it in a cup of water and put it in the microwave for a min. Take the pieces out and dab them with a paper tower. This will allow them to sink.
I hope this helps.