What is this?

[uvavi8r]

I have a 1.5 year old fiberglass pool, equipment is in my signature block. This week I noticed some white/silver circles in the pool...they don't rub off using fingers (I haven't tried anything else). At first it was 2, now it is a dozen or so. I also noticed (today) what looked like sand in the bottom of the pool. I vacuumed everything out and captured the debris...couldn't see what I thought was sand in the pile, but there were a bunch of these little seeds (about the size of a pine nut). Does anyone know what these seeds could be (I have black walnut and oak trees around my fence)? Could these seeds be causing the stains? What should I use to try to clean these stains off?

Thanks in advance

Seeds

Stains

 

[uvavi8r]

One other thing all of the stains are pretty much just like the one in the picture, and they are sort of concentrated around one area of the pool (and a return jet).

 

[Texas Splash]

I'm not sure which tree those are from, but seeds can quickly leave small tannin stains on a pool floor. They can be dark and sometimes have a small tail like a comet as they slide. If you have a chlorine tablet laying around, you could try rubbing a stain as a test. If you don't have a tab, try rubbing a little cal-hypo powder on it as a test.