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Green algae can be many different things but it is often a sign of excess nutrients in the water. If the excess nutrients aren't reduced the algae may overgrow desirables including coralline algae and even corals. Try to better describe the algae or post a pic if possible. In the meantime, test your tank's water for nitrates and phosphates and also post some detail about your tank...lighting, how long its been running, filtration etc. HTH