That's not the platys or barbs: that plant is melting away, and they're snacking on the decaying vegetable matter and algae. In fact, I don't even think that's an aquatic plant: it looks at most like an emersed plant that can have its roots in the water, but the leaves need to be above the surface. Way too many stores sell these plants as "aquatic." I would remove it and replace it with a genuinely aquatic plant.
Do you remember what it was called at the store?
2x20-longs, one heavily planted. Platy rancher. They. Won't. Stop. Breeding.