Having a hard time finding photos with a similar pattern. To me it looks fungal, really would prefer to not visit a vet, haven’t seen many that seem to really understand tortoise care.
I have noticed recently that a 30+ year old female Desert Tortoise I care for has developed a mold like pattern on/in her shell. She is mostly kept outside during Spring/Summer in the southern United States. During Winter/Fall is kept indoors with a heat lamp and usually in a large concrete...