Yep, if the green color is in the water itself, that's what we call an algal bloom.

How thick is it - at what depth does your hand, a white object, or (better yet) a Secchi disk disappear? 12" to 18" is usually considered optimum. Less than 12", we worry about an oxygen crash resulting (when the bloom dies or when cloudy, windless days occur).

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
[Linked Image from i.pinimg.com]