When an Acid Hag or Lady Luna are spawned in water (usually a large body of water), they leave acid but the water immediately flows back in and displaces the acid, making it drop as a block. I know this won't happen normally but still it should be changed in my opinion.

How about having these mobs check to see if they are in water before dropping acid? Not sure how easy this is, but if it is easy, it's worth a shot.