Dirt or pollen is like anything else that's not soluble in water.....if you stir it up, as with the vacuum or Aquabot, then let the water get still, what didn't go into the filter will settle back down in the bottom.

However, if you're sure it's not dirt, you could try swimming down to a particularly dense area of whatever it is, and use an eyedropper to get a sample. By putting it into a jar or glass of water, it might be easier to identify.

Are you using chlorine overnight? By that I mean have you tested your Cl at night after the sun is off the pool, and then tested again in the morning before the sun hits it? That would give you a true idea of how much chlorine demand you actually have....and if it's none, then the stuff is dirt. If it's algae, you'll see a dramatic drop in the Cl level from night to morning.

Janet