Small, slightly discolored bump on salamander’s tail

[brischutte]

Hello! I’m new to keeping salamanders but I’ve done a ton of research. Currently he’s being housed in a 10 gallon with a mix of eco earth, potting soil, and reptisoil. He has 4-8 inches of substrate on almost all parts of the enclosure so he can dig. I also have moss in his enclosure but not much. I saw that sometimes moss can cause ph/acidity issues, is that what it could be? I feed him crickets and mealworms 3 times a week and once a week I powder them with calcium, sometimes with d3 sometimes without. I spray down his enclosure once or twice a day and his soil is bio active (has springtails and dwarf isopods). I plan on upgrading his tank to a 40 gallon in the next couple weeks. He eats really well and is always excited to. He sheds fine, is active and interacts with me a lot! I love him very much and I’ve searched these forums for about an hour trying to find out what’s wrong but I’m still not sure. Am I keeping his soil too moist? Is it a fungal infection? It doesn’t seem to bother him but I don’t want to be too late if this is something I need to go to a vet for. I didn’t sift out the potting soil I added to his substrate, so could it be an abrasion from him burrowing? I’m also worried the soil might have chemicals in it that are irritating his skin. I’ll be able to move him out of his current tank really soon. I’ll only be using eco earth and reptisoil, and then a bio active substrate that’a free of any chemicals in his new enclosure. Should I wait to see if it goes away once he moves into his new enclosure? Or should I take him straight to the vet?