That pic is a perfect example of why I won't use fleece bags for hay, lol. My only suggestion is maybe get a fine toothed comb and try to comb as much of the hay off as you can. Or if you want to wash the pee off first I would just handwash it in a bucket with some vinegar and water, then dump it outside when done so you aren't dumping hay down the drain. Then you can try combing the rest of the hay off before putting it through the washer.