Just a little mold :) this won't hurt the plant
the plants are just staying wet a bit too long.
This is waaay more likely than a fungus conspiracy.
change the wattering schedule or the conditions around the plants ie soil, light, or temp.
This is an indicator though, something should be worked out or root rot could develope.

Agreed. The soil looks very anaerobic. Like a bag of water when wet, like concrete when dry.