Hey! I don't think that's sphagnum, I think it's that green moss. Sphagnum comes in thick strands and can be white/cream colour. Sphagnum is safer because of their thicker and longer strands are hard for the frog to swallow and it holds more moisture and so pats down better.

My fire bellies aren't the best at catching stuff because dont use their tongue to catch prey. Are you providing a light that gives out UVA? It should also give it low levels of UVB. A 5% tube or compact would be needed for that size of tank.