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 bottomed enclosure with pine straw or aspen shavings as substrate(have never seen or noticed she eats these substrates in her poop). When she is indoors she is frequently taken out in the sun on nice days to graze on weeds/grass and to get some UV rays. Diet is mainly collards, kale, spinach, romaine, mazuri tortoise diet, Timothy hay, and they very occasional strawberry or raspberry. Any idea what could be causing this and how to treat it?