Congratulations, you have an algal bloom.

That's a good thing, as long as it's not too intense. 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).

For info on measuring clarity with a secchi disk, see here.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
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