I had this in my pool upon opening last year (tiny crystals all over the floor and walls , but only under the waterline; come off easily, but pain to clean up). It's probably calcium leeching from your plaster, probably due to too low calcium levels before closing, among other things. (What was your CH levels when you closed?). I have read recently that it's probably caused by a combination of 1) low CH levels, 2) high pH, and 3) cold water temps. Also, your current CH reading of 250 might be inflated, because you had this calcium from the plaster now in your water. Before I closed my pool this previous season, I set my CH level to 450 (also making sure my CSI was in good range). I just opened my pool a week ago (to a CH level of 275 - expected since I have a mesh cover), and no calcium crystals formed this year, so keeping the CH level high (in accordance with your CSI) at closing seemed to work.