why is my water turning green?

Have you tested your water lately? If so, what are the results ... and if not test it and post your results. High nitrates can easily kill inverts and corals. Also, test your tank for stray voltage, that can also kill starfish quickly.
Green usually indicates an algae bloom. Algae blooms are typically fueled by high nitrates and/or phosphates. High nitrates will kill your inverts. Start by looking there.
Yep. Water changes. But if you are using tap water, then water changes probably won't do much good. In fact, tap water often causes the problem in the first place, so be sure to use RODI water.
How did this turn out?

If you've got caulerpa in your tank it can send it's chloraphyll into the water if it is in trouble. If you do have caulerpa and it's turning white, pull it.