Mildew! Usually a sign of extreme dryness at the roots. Give it a really good soaking and spray with 9 parts water to 1 part whole milk.

Or any milk,so long as its cows milk - I use skimmed cos that's all I've ever got...

Bamboo do you know why goats milk won't do?

Dunno - no one really knows for sure why cow's milk works, they think its something to do with the lactic acid.