What is it and how to kill it?

See pic.
A fungus or something on my house plants.
How to destroy?
Thanks -

 

If the underside of the leaves also have little cottony tufts and a sap like residue, you have mealy bugs, but it's hard to tell by your photo. I would first get rid of the decaying matter at the base and while you do this, watch for tiny insects (crawling, jumping, or flying.)

Most plant pests can be treated with a diluted spray solution of dish soap and warm water. Do this twice a week to keep your plant healthy. Depending on the size of your plant, you could take a cotton ball of the same solution and wipe each leaf removing the sap. Also consider adding those little yellow sticky traps if you cannot ID any particular insect.

If you're still unsure what the cause is, take a leaf to a local garden shop to ID it.

 

Thanks KT!