Susan B <firstname.lastname@example.org>
Scott mentioned verminiculte holding water but perlite does too.
Years ago I had a small plastic bin of perlite, about 9x12 inches (sorry Wilbert) and about 4 inches deep. In a fit of absentmindedness I poured some water into it instead of a potted plant. As an experiment, I dumped out as much water as I could and then left it in the summer sun, figuring it would dry since the perlite is so porous. Well all those little pores held water and that perlite never did dry, it did develop a nice green mold which grew nicely until I threw the entire mess away.
Ever since then I've been leery of mix recipes that call for a lot of perlite to keep the mixture from becoming too moist.
Perlite does grow algae...when heat and moisture are both high...setting the container in the sun only promoted the algae growth...you shouldn't have left it in the container...should it happen another time...spread it out on a tarp and leave for a day or two stirring constantly...Algae is not a bad thing as it helps to add nutrients to the soil and letting things dry between waterings shall help to eradicate the algae...
I've many plants which have added perlite in the mix...and when I notice algae growing I cut back on water and make sure it dries between waterings..
if you can salvage the old perlite do so...you can spread it out to dry in the sun or set it in the oven at lowest heat for a couple of hours...may not smell great but why waste the perlite??
kfb maine zone 5