I'm having the same problem with my ag pool. It started about 2 weeks ago. I got in to vacuum as cold as the water is this time of year and after I ran the filter for about 2 hours, I saw the same brown algae-like substance on the bottom but mostly in the middle area. My water is also a cloudy blue after vacuuming. I'm going to try doing a super shock but would like to know how much de to add to my sand filter. This stuff definitely seems to be blowing back in. When I backwash after vacuuming, there is only clear water coming through the hose. I'm also going to try using a filter saver (nylon skimmer sock purchased from In The Swim catalogue). Hopefully, if I vacuum with this sock in place, it will catch some of that dam BROWN STUFF. I've had an ag pool for the past 15 years with no problems but the past 2 years have been a nightmare. I didn't know that you could add de to a sand filter.