

Is this mold? Central air humidifier filter. Didn’t look like this last year.
Basically the title, changed my humidifier filter and this is what I found. The substance in question was dried and crumbly, though. Unsure what it is. I cleaned the containment with a 50/50 water and bleach mixture before putting the new filter in. Humidity in house is kept between 45-55% 99% of the time.