Something's a bit weird.....

I have never seen the water in our millpond look so green! I'm a bit worried about it really. Sometimes it gets a bit green with algae in long spells of hot weather but this is quite different... almost as if someone has poured some green paint in it! I took some out and looked at it in a glass jar and it looks ok close up. Perhaps the martians have landed in it over night!!! ;-) You can see it here in it's more natural summer green.

Any ideas anybody? Is this a common September phenomena that I have never noticed before?