This is also true for mold. Mold is also a fungal growth. There are many different mold species that come in many different colors. Mold multiplies by producing microscopic spores that are like plant seeds. Mold spores can take hold, grow, and cause health issues and other problems to your hot tub.
It’s not uncommon to occasionally see mildew or mold above the hot tub’s waterline or on a hot tub cover or pillows. It likes to grow in warm, humid environments that don’t see the sun, which sounds like everything under the hot tub cover!
Mold reproduces by producing small seed-like spores. The spores look for porous surfaces, like the hot tub cover, pillows or filter.