That's not comparable types of pollution. If it's something we put there on purpose and provides us a benefit, I'd say even use of the word pollution is incorrect.

I don't think there's a big micro meteorite problem at the Lagrange point, but if there were for arguments sake, then you need to plan for it and have a way to replace shades that get knocked out of position. It's not like the whole system would fail if you lose one shade. That's what engineering is about, planning for all the likely eventualities.