From several decades of dealing with shell fungus issues.
Those divots don't look like they were caused by fungus. And fungus is very unusual in Sulcata.
Keep an eye on it for a few weeks. And if there is no change. Relax.
I think you could soak him in warm water, then scrub the spot with a toothbrush (and even a bit of white vinegar if it's being stubborn) to clean it up ....