Most of all...
Is there any other symptoms ?
Excess of slime in peculiar. Gill discoloration. fins erosion. Sores ???
Something else changed in their comportment ? Anything abnormal. hanging somewhere, fin clamped ?
Some hobbyist reported that their cories where not that fond of the sand they been given.
Like always when the fish is the same color as the substrate on pictures it's nearly impossible to see anything.
The cories looks ok but the mollies white patches are something that would worry me (that doesn't look normal). It may be part of the problem.