ignore the mildew...and I seriously doubt it is powdery mildew as I have never seen Hosta affected by that. I would suspect that the mildew is actually another bug. Go and press it and see if it trys to jump away from you. These bugs are annoying, but they don't really harm them significantly.

As you have noted, hostas are slug food. The beer works well, there's also slug bait (but don't use it if you have critters who might eat it). Some people swear by crushed egg shells around the base of the plants, or diatomaceous earth or copper strips. I've used the eggshell, and it might work....frankly, I just ignore the slug holes. Your plants will get bigger and you won't even notice them after a while.