How to clean microwave turn table

Try WD40 or Goof Off with a steel wool pad.

If it is glass, something isn't making sense about the glass being yellowed. Is it a build up that can't be scrapped or chipped off?

I'd spray the turntable with Krud Kutter, a proprietary cleaner/degreaser, you can purchase at Ace Hardware stores and then allow it to soak in a white vinegar and water solution. The vinegar and water solution should remove any leftover cleaning product and has a mild bleaching action as well as dissipate old grease smells. I'm intensely cheap when it comes to commercially made cleaning products (normally if ammonia, elbow grease, vinegar or baking soda won't clean something...I use more elbow grease, LOL) but I discovered Krud Kutter when ordering supplies for multiple dwellings and IMO it's worth every penny.

HTH!