Is this shell rot on my yearling Eastern Hermman's plastron?
I noticed it when I was soaking him today. It wasn't there yesterday, or at least wasn't nearly as noticeable, because I give him a once-over each evening.
I don't believe that's a fungal issue.
Those look like superficial rub marks.
Give the area a good lookover every few days and look for improvement or worsening.
Also, check your enclosure for any rough surface that might've caused this.
It's also very possible that another member with better eyes than mine will see a detail that I've missed