Why are they dying?

[ClownFishGuy6]

I have 1 zoanthid. I don't know what they are called but they are dying. Could it be all the green algae on the base of the rock that's creeping up to the top? Because my tank also has a very big green algae problem.

https://gyazo.com/e612a94328b8ed5532a68add9abffac2

gyazo.com/ca8117e212faa270a999682f502a17a0

https://gyazo.com/0df75360c48168b57a0683276318fd79

And in that last picture can someone tell me what kind of star fish that is? It just kind of appeared in my tank.

 

[Rhoms]

The star fish is called a asterina starfish and could be the reason your zoas are disappearing, they are known to eat them. The algae could be either a lack of flow or the tank cycling. How long has your tank been up?

 

[Sk8r]

Asterinas can be a problem with zoas.
Likewise, corals don't like phosphate or nitrate that's excessive.

 

[ClownFishGuy6]

Its been up cycling for but 5-6 years