My pool with white plaster is about a month old. I have noticed when it is overcast that there is a browning discoloration look to the plaster. It looks like there is dirt on the bottom of the pool. When it is bright you don't see it. This started in the shallow end but is making its way to the deep end. My test for minerals was negative and I keep the other levels well within range. Is this a normal occurrence for white plaster?