Pretty far away to tell what that is, but what substrate is this snake kept on? It's very likely just cage debris under the scutes (belly scales) that trap some dirt &
shows up because your snake is white. Snakes use their scutes to dig in & establish traction crawling forward...it's pretty normal that they get a little dirty in the process.
Rudeness is the weak man's imitation of strength. Eric Hoffer(1902 - 1983)