Here's something you may find interesting.
It's been about 15 years since I've had my last pool so I was a bit rusty during my install and initial maintainence this year. I had what I thought (and kind of still think) is "sand" at the bottom of my pool.
However, I did have a few good reasons:
1. I bought my pool (2 seasons old) from a friend who's kids refused to swim anwhere but their swim club so it was not used at all other than a few times. I changed sand inthe filter anyway and noticed that whoever set up the pool originally used a few inches of pea gravel (which I used to use in larger filters myself) , in this current (100lb) filter. I thought possibly the better flow around the laterals in such a small filter may have cracked / shifted something and may be contributing to my issue.
Perhaps in my particular situation I may have had some channeling while this pea gravel and sand settled. I did a good rinse and re-used it but probably should have done without it in such a small filter. I'll take care of it before I open next year.
2. When I filled the new Pool Sand, I mistakenly filled it to the "seam/ groove" in the Hayward filter thinking it was the 1/2 way mark. It's actually over the 1/2 way mark in the tank (which kind of hit me one night just starring at the filter) and since the instructions state to fill the tank 1/2 way, I thought that between the pea gravel and the "less sand" coupled with the fact that I have a bit less space for water in tank, All may have been contributing to my problem.
Over the course of the last few weeks, I've vac'd (and keep in mind, the "sand" does distribute all the way to the other side of the pool but I'm not sure if thats due to me tracking it around while it's suspended and I'm in the pool).
Still . . . I kept thinking: "could this be some small amount of "silt" (I'm in a wooded area) or some type of pollen, or is it actually sand). It is gritty just as you state and I'm sure some of it is indeed sand.
However, when I run the filter at night, it does seem (much less now) to be distributed all over the pool. SInce I have a Armor Shield Pad under the pool, I have a very slight ridge that you can feel with your foot if you try hard and the "sand" will seem to collect the entire length of the 33' along this seam. (Which I would think plain old sand would not do).
The short story is I did do an overnight test, I did shock and did really backwash and rince my filter quite a number of times. Sometimes every 2 days and sometimes I backwashed a few times in a given day.
I think Waste has a really valid point. Even though it may be gritty, it may be some sort of algae. I never thought about it until I read his post but if I slowly push the brush, it does move but I DO beleive that it does also cloud and disappear.
That's a great tip and a really "common sense" answer that honestly never really crossed my mind. Thanks Waste.
In my particular case, I think I have a slight pollen issue BECAUSE... since I put a nylon over my skimmer basket, the amount of "sand" or whatever I thought it was had been almost eliminated and let's face it, Algae just doesn't "go away" (especially since I have not really done anything so to speak to get rid of it if it was algae).
It could also be that i've cleaned and flushed all of my sand , pea gravel etc, god rid of my high level of sand etc through all of these backwashes, allowed everything to "settle in" or whatever but I thought I would just post my particular circumstance for others to see.
I think Waste has a good point. It may look like sand but could very well be something else. Especially if it clouds and disappears. In my case (although not algae - I think), it clouding up and getting trapped in the nylon sock is what helped cut down my issue.