It's probably storing cum particle systems on the futa part (which burst when panties are taken off).
This was a big issue with previous version, i fixed it in 1.4, not sure why is this persisting. Are you using latest (1.4) version ?
To clear the stains : you just need to bring the Dirty stat to 0, that can be done via Public Toilet, Cleaning spots in nest, hActs with females or 'asking nicely' a fairy ;)
This continues during that too and yes i have 1.4 it might be due to the lower quality version maybe haven't tried going back to the higher quality version to test it out cause it also lags a bit if I try I'll probably try later cause I'm getting a better quality gaming PC soon and I'll see if it continues then probably going to have to play for extended period of time to test though.
Will note if I disable futa mode it stops and I don't usually wear the panties unless it requires me to wear it.
