Need some help with a new problem we are having. We have an above ground 18 foot round pool and a de hayward filter. We just had a new liner installed and refilled with fresh water this spring. For the last month or so we have been noticing white sandy deposits on the floor of the pool. When we asked at the pool store they told us it was most likely mustard algae. Our water is crystal clear otherwise and its in full sun so its been really warm this summer. We treated with shock and algaecide as instructed but now the problem is worse than ever. Im not even sure if it was ever algae. Today I sucked up the 'sand' with a turkey baster and put it into a cup. Looks like white granules that settle. Also, although we have been backwashing, it seems as soon as we put the filter back on the strong circulation makes all this stuff settle in the center. What is it? Anybody have any ideas??