If this just started looking this way it's most likely shell rot (fungus). Wipe it down with Betadine and apply daily athlete's foot cream (Lotrimin or Lamisil, or whatever your drug store has). Don't worry, this will be ok, just don't let this baby sit on wet substrate. RF's are prone to this. My babies have not had it (so far!) I've had them since June, 19 of last year.
I am not sure if this is shell rot or not, I did a little research and I am now thinking it may be where his egg sack was not fully absorbed? He is very small, definitely under 3 inches and he has had it since I brought him home. This is my first tortoise so I am not sure how to tell if it is shell rot or his egg sack scar
When babies hatch they have a tiny little sharp "tooth" on the end of their snout. They use it to slice open the inner membrane of their egg and crack open the outer shell so they can breathe and eventually get out of the egg. If your tortoise still has this, it would indicate that he is very young and the material on the plastron is indeed left over yolk sac material. If there is not egg tooth, then you might have some shell rot starting there. Do an internet search for hatchling tortoise egg tooth.
Is your substrate wet all the time?
I will always defer to more experienced RF keepers here, but I'm trying to offer some general tortoise insight here.
I have coco coir on the bottom that stays pretty moist but I have cypress mulch on one half of the tank that isnt moist. He stays mostly on the cypress mulch at all times. The breeder only used cypress so I think that is why he prefers to stay on the cypress.
it is nothing more than the yolk sac shrinkage and bottom of the plastron healing on a young tortoise. Egg tooth disappearance can vary and there is no really "set time".
it will be fine,,,,,just watch your moisture content in "enclosure medium" . Some hatchlings are born with larger yolk sacs and take longer to "develop full seam" even while say.... a sibling from the same hatch will be already healed up.