When you go to the tech screen, and click on either of the pollution filters and then hover the mouse over the little green recycle icon in the Effects section, it says 0.9 x Unknown key: "recipe-name.kr-air-cleaning-2" (Recipe).

I'm sure it's an easy fix for someone who knows how to mod...but I don't.

A guy at the Factorio Discord server, channel #modded-chat (I don't have his permission to name him) told me how to fix this.
Go to the mods folder and look for the zip file k2-space-age. Depending on your text editor, you may have to unzip it before editing. Then go to the locale/en sub-folder, look in the Krastorio2.cfg file and look for the [recipe-name] section.
Add these two lines:
kr-air-cleaning=Pollution Filter
kr-air-cleaning-2=Improved Pollution Filter
Save the file, then exit out of the editor. If Factorio is still running, you'll have to exit all the way out of it.