Melonite & Tennifer Finish
Melonite/Tennifer is a Top Coated finish.
Removing the original rear sight & the new installation probably abraded off a bit of the top coat.
Something that could most likely not be avoided.
It's very doubtful that the underlying Melonite "wore off" since it is a "metal treatment" rather than a surface finish.
It's extremely abrasion resistant as well as rust and corrosion resistant. ~ Melonite being tougher than hard chrome.
If looking at the wear area bothers you then do something about it.
If the appearance of it does not bother you then I sure would leave it alone.
Remember that if you do want to have it redone then you only need to send the slide and that can always go by regular insured mail.
Remember also though that in order to refinish the slide...whoever does that work will likely need to remove your Tritium front and rear sights again & then reinstall them. :yup:
Some things are probably best left "as is" unless a little wear spot bothers you visually.
The more somebody screws around with popping your sights off & then back on again the worse off you'll be.
Just my opinion.