My Greek tortoise is 4 months old and recently I've noticed some white squiggly lines on his carapace. I am not sure what it is and am hoping it is just hard water stains and not shell rot. Some background info relevant to my concerns: his substrate is coco coir that is damp underneath and dry on top, and he receives warm soaks everyday for 20-30 minutes.
Here are some pics of his carapace and plastron:
Here are some pics of his carapace and plastron: