Glad to see that it arrived well. And sorry for sending you that messy moss. I didn't realize it was so full of clado.
The stuff that's in there won't harm anything or spread very much, but shrimp and snails rarely eat it. I wouldn't risk shrimp with a betta. Algae killers should handle it if you are bothered by it, but take out the nerites to be safe unless you know you have one that won't kill inverts. I treated it (and washed it) before I sent it, so it should be on the tail end of life.
Ammonia is a form of nitrogen that plants will consume... so ammonia poisoning doesn't sound likely. But whatever it was, the problem seems to have gone away.