I had a rooster with a lot of white runny clumps stuck to him that I mostly ignored. However, when I gave him Sulfadimethoxine after a few months (I was treating him for another problem), he acted like his guts were in agony, & I now suspect that he'd been experiencing something like Ulcerative Enteritis & the sulfa med caused extra pain when it contacted the ulcers.
I soothed his guts by feeding some buttermilk for a few days. I also gave shots of Penicillin in case it treated whatever kind of enteritis he might have had.
His diarrhea cleared up for the first time in months!
I don't know for sure if it was the Penicillin, or maybe even the Sulfadimethoxine that cleared it. Or maybe the combination??
But now I do pay more attention to whitish clumps on rump feathers in case a bird may be suffering from something potentially painful & treatable.