Well....I sure know spiders must poo but i have never been aware of it....imagining it's some other sort of critter....but soap and water should remove most stuff.

I think soap and water will work. I wipe it off of hard surfaces with a damp rag.

Lotta spiders in my house, under the cabinet toekicks. Specks of poop collect on the floor (it kinda blends in to the speckles in the tile. I clean it off with a bit of Dawn dishwashing detergent on a damp cloth. Regular washing should handle the curtains. Extended soak time is safer than aggressive agitation. An enzyme additive such as Biz powder may be helpful. Be watchful of fading if colorfastness is in question.