I wouldn't check its mouth just yet. Other sings are visible mucus in and around the mouth and nostrils, and you may see it smeared on the cage walls or hide. You may also find it laying with its nose pointed skyward as this allows them to breath easier when congested, note some snake just like laying like this as my BRB does it often.
As for the mulch I buy mine from Petco and use Forest Floor. I like it because is is well shredded and is a great quality and consistency. It is more than the garden quality, which I have tried, but it is so much better.
I like to keep more natural looking tanks and actually mix Forest Floor, Eco Earth, and Repta Bark together to make a nice earthy looking blend. If holds moisture great and used together it keeps the Eco Earth from being dusty. I mix it to the look I desire in a big tub and then place it in the cages as needed. I even started using it in my tubs with my younger Bloods. When it gets to dry I pour water in and stir. Other than that misting keeps it pretty well hydrated.
The mix is also still good for snake that like to burrow like my Hoggy does. She can root around and dig tunnels all through it.