Thanks all!
It was definitely from the feed. I left the frisbees out for one day while I fed in the coop, and when I retrieved them the next morning, one had been dumped over and there was more white stuff growing exactly where it covered.
I washed the frisbees and moved them into the coop. Ever since I've only been feeding in there, and no more white stuff on the ground. I guess they must have been knocking some onto the ground and not eating it all, and it grew mold. I'm so glad I asked, and glad I moved it.
It was very convenient to feed them that way, but ...
I'll have to be getting new feeders and waterers anyway. I really want to move the water OUT of the coop, for dryness sake. And I really want to find a feeder that they can't jump on top of and mess up from above.
Now I know what to do with that 50# bag of lime my boss gave me.
Every once in awhile that stuff just grows on the grass anyway, but fortunately not in the chicken run if there is no food there.
Thanks again, all!
trish