This heater duct is blowing directly on the concrete. It is causing moisture which is bringing what looks to be a mold-like substance to surface or is it efflorescence? Testing can tell.
How many filter are in your home? Clean/replace filters in your home as needed. Use This list to get started: EXHAUST FOR STOVE ( MICROWAVE) HEATER UNDER SINK WATER SYSTEM CLOTHES DRYER/FRONT LOAD WAS