The white stuff looks like white fly eggs/poo, which I'm hoping the ants are eating. Does it grow fluffy in places, like cotton candy that didn't get swirled right? If so, that may be what it is. I don't know if they cause that rotted looking damage, though.

The treatment here was blasting the flies with the water jet for several minutes each day to destroy the eggs. Took a while, but they're gone, finally.

Roxy, who has lost a couple of hibiscus to white flies (where the plants did grow a black mold in addition to the white fluff)