Welcome to the forum! We are glad you’re here.
In tortoise terms, the crack is between the bottom and lower right scutes. As if perhaps some keratin has flaked next to a growth line? Nothing to worry about, in my opinion but let’s see what Yvonne thinks.
I’m sorry to say it but there are quite a few problems with your setup, the most significant being that it is too small. The wrong setup and lighting can have quite an effect in your tortoise’s eating so let’s take a closer look at that.
Please take a look at these threads and come back with any questions you may have:
http://www.tortoiseforum.org/threads/russian-tortoise-care-sheet.80698/
https://tortoiseforum.org/threads/beginner-mistakes.45180/