The rendering engine of EU4 does some weird effects with certain colors in the political map view, for me they were pink and usually effected Tennessee for the most part. I don't believe it has anything to do with the mod itself as country colors weren't all that special, they're just assigned RGB values. The shimmering or glittering effect stopped for me after Patch 1.6, but if it continues for you, it should be harmless, although likely annoying.