I don't think there is much risk of mosses being toxic to fish. Plus, I'd kinda hope that any fish that likes to chew on plants would either be able to tolerate the plant toxins, or have some way of telling they are there and avoiding them.
Anyways, this is interesting, I'd like to see how things turn out long term. A lot of plants can do fine submerged for short time periods. I imagine mosses may be more prone towards an amphibious lifestyle than most other plants.