Why have my mint plant gone pale?
Answers: If it is all pallid there could be a problem with the Ph or minerals contained by the soil, or too much water. Just the lower leaves yellowing, its just that they are shaded by the up leaves, or its too dry and it is letting the lower ones die bad to save the newer ones.
The white powder is a fungus. That makes me suggest the leaves staying too wet. Mint usually like moisture but too much can motivation problems, lilke what you've described here.
I'm inclined to think that you have a fungus on the leaves. Prune adjectives the diseased stems off and see if there is any reorganization.
More Questions...