I found the cause last night. It was a dying/dead tiny horned nerite. No clue what happened to him.
The ammonia was at 4.0, which was way too high to just leave alone. The poly filter, as I understand it, does not remove ammonia, but sorbs it. So it makes the ammonia still available for the cycle.
Ammonia was 1.0 last night and I left it alone. This morning the reading is 0.25. These numbers I am not so worried about since the source of the problem has been taken care of. I think I will just not keep snail in this 2.5 gallon tank once I have my 5 gallon going. I was planning to transfer my big nerite (who is a pig eating all the algae) to my 5 gallon and to keep a small horned nerite in the 2.5 gallon tank. But for whatever reason the little horned nerite just didn't thrive and ended up polluting such a small tank.
There is algae growing in the tank, but as soon as it grows nice on the rocks, the snail comes and eats it all up.
There is still green in the back area of the tank that isn't easily seen from the front and my lava rock is starting to get some green back. I am glad the snail is grazing on the glass again so the rock has a chance to get more green.
If I could go back in time, I would have used lava rock substrate with minimal sandy areas so that most of the poop can fall between the rocks and be out of sight. This is my plan for the 5 gallon tank as soon as I find a good source of safe lava rock. I have not had the chance to visit the local Home Depot yet. My 5 gallon tank is going to be more rocky.