The answer is sort of sometimes, but not with polywatch, I've had it done by my watchmaker to some degree of success but it requires special abrasives and a tool, often cheaper to just buy a new one as they're not hugely expensive for the Mark 2.
Yups, it is mineral glass , not sapphire so I think I see OEM crystals on eBay all the time for a "cheap" price relative to most Omega sapphire glasses. Judging from just the one picture, I would keep it as-is since it matches the character of the watch. But if it bothers you, then go for it!
This glass is difficult to change, even for trained omega watchmakers and they often brake it.. I paid 150 to get one changed by omega... no so expensive...
You just have to have the right tools, and it's actually not bad at all to change. You could use cerium oxide or diamond paste, but honestly it's not worth the time and effort involved.