Tough to say without seeing but.......
it's either mold or some type of algae or mildew.
Can you replace the foam in the humidifier?
If you can, go to your local craft store and buy a brick of florist foam also called "Oasis". Oasis is actually a brand name for the stuff and if you can get that brand all the better, but not critical.
BUT BE SURE that the type you get is the "wet" type. There are two types. One is for using with real flowers and is the wet type that you want. It holds :smiley35: loads of water. The "dry" type is for use with artificial flowers and won't hold water much at all.
THEN...go to your local cigar shop and buy a humidifier solution. It contains a mix of 50% distilled water and 50% propylene glycol. The PG will do two things. It will help stabilize the RH% to around 70% by itself and it will also prevent mold or algae from growing in your humidifier.
Cut a piece of the foam to fit your humidifier and soak with the solution.
Knock on wood I have never had the problem you are experiencing and I have often used foam and plain distilled water. Hopefully you can clean the plastic humidifier unit too. Just use hot water alone and a clean paper towel to scrub away any slime residue.
There are other alternatives to using foam as an element. You can use well rinsed cello sponge or one of the many forms of silica gels or polymer crystals on the market. I am currently using a product called "Soil Moist" which is a polymer crystal used to add to potting soil to help retain water. Of course you don't add any soil to your humidor! :smiley36:
Alot of the guys here are using Climmax humidor beads which work very well. Do a search for "beads" on this board and I'm sure you'll find a lot of info.
Good luck!