I was wondering if anyone knows why brown spots are growing on my fake plants and how I can get rid of them. They wipe off of the plastic plants but won't come off of the fabric ones.

The brown spots are most likely "brown algae" which is a type of diatom. It can be caused by excess silicates in the water as well not enough light.

The only way to get it off is a really good scrub. Maybe use a toothbrush?
Mary

Or if it is a really big problem and u can't get it off with scrubing, use diluted bleach to wash it, but rinse it off very well.