Fiberglass keg lid?

I got a refurbished corny keg and it has a dark gray fiberglass or injection molded plastic lid. Has anyone used any like this? The main thing that's concerning me is that the bottom of the lid (that would face the beer) has some discolorations, that almost look like dirty spots that won't come off. Are these lids okay to use or am I risking infection? I've soaked in Oxiclean and scrubbed, but the spots still aren't coming off.

I have one like this. It's a blue plastic lid. I don't think it risks infections but on mine the relief valve leaks and I can't find one to replace it. I just use if for fermenting right now and accept the broke relief valve as pseudo-airlock.