Can rain deliver mustard algae? If not, then I'm now 100% convinced that what we have had since last summer is not MA. We just had some really nasty storms last night and a this morning. I went out to drain and bleach the pool (didn't have any bleach last night for regular treatment) just now and found the exact poofy stuff on our newly installed solar cover . It poofs away just by swishing the water, just like in the pool. And there's a TON of it all over our cover now.