Tecnics 1200G platter cleaning

"One man's dirt is another man's patina."I would just wipe off any dust and let the brass age gracefully.

Try this: https://www.flitz.com/brass-copper-tarnish-remover/ 

Dirty or oxidizing, there is a big difference. If it is oxidizing, chances are that it is not clear coated, therefore, you will need a metal polish.  Try polishing the platter's underside as a test before moving on to the top surface.

Mine still looks like brand new.

i am thinking its carbon graphite from the boston audio mat but that does not explain what looks like tarnishing around the edges of the platter