Its somewhat normal.. it doesnt look extensive at all either so I wouldn't sweat it. You dont need super high humidity now that you have an established plant going, Id do away with the tubs of water at this point. Unless you are dipping below 40% or so with the humidity. Its pretty harmless tho so dont fret it, theres all types of bacteria and stuff in your soil - it just shows up clearly on the perlite cuz its white