Hi and welcome

Some tips for your baby...

I would purchase either a infared heat bulb/lamp with a dimmer for overhead or a UTH heater for your land side. That will help control your tank temp and warm him up. It will increase his metabolism and make him hungry .

Firebelly toads LOVE the water . This is normal for them. I would have at least 1/4 of the size of his home as water. Be sure to change the water often. A filter can help, though water changes are still necessary.

Remove the moss... You are correct. It makes it hard for your little guy to catch his food and also is a big risk. If eaten, it can block his digestive system and can be fatal.

Get your baby a friend . They are very communal and love being in groups. If you can purchase one from the same group you bought yours from you can put him right in. If not, you'll have to quarantine them for at least 4 weeks to prevent illness. For a 10gal tank though I wouldn't put more than 2 in. A general rule is one frog/toad per 10 gal. I think you could get away with 2 if you change your substrate monthly and your water daily with no pump and weekly with a pump (approx).

Try covering 3 sides of his tank. It makes them feel safe. There are many options. Simple, quick, and easy...fish tank backgrounds. There are vinyl ones that are put on like window clings and they have outdoorsy patterns too.

Provide some plats for your baby to hide under. Makes them feel cozy and safe. It will help him to turn more green than brown too.

Substrate...coco fiber, such as Eco earth, or plantation soil are best. Avoid anything with bark-like pieces, as they pose the same risk as the moss. You can use gravel or rocks in the water area, but not in the dry area unless they are large enough that the toad can't accidentally ingest them when eating their crickets. River rocks are generally big enough.

Hope this helps .