I just discovered that there is a sandy-looking substance in the dirt near our waste line. We just got done doing a SLAM so we have backwashed and vacuumed to waste on several occasions during the past week. Could this sandy-looking substance be dried up mustard algae or is this a sign that my sand filter is leaking sand when backwashing? I don't see any sand inside the pool and the sand filter seems to be working efficiently since our pool went from dark green to clear in 5 days. Sand filter is probably 15-18 years old and it is a Hayward Pro Series S244T. If it sounds like this substance could be sand, I need to know if it is safe to continue running the pump on filter until I can get someone over to look at it. My local pool pro is busy with pool openings right now so it would probably take a week or more to get him over here to look at it.