Is my live rock dying?

I have a 20 gal tank with about 15 lbs of live rock. I got a couple new pieces about a month and a half ago and I think they might be dying. First they started to lose their purple color and turned whiteish. Now they are turning brown. I have 2 clown fish, a royal gramma, and 5 hermit crabs. I change out about 10% of the water every week and keep the algae off the walls. What's causing the rock to turn brown?

I attached a picture of the piece. (you can also see one of the tunnels my royal gramma dug)

 

I see a lot of purple... does that come off easily? If so that is Cynobacteria, and that needs to be corrected.

 

Live rock doesn't "die" in a healthy tank. What is dying is the coralline algae which can be caused by low calcium levels or just the wrong light spectrum for that particular strain. If you have coralline growing on other rocks in your tank, it will spread to those soon enough.

 

how old is the tank? when was the rock added? did you cure the rock before you added it?

 

Tank is about 8 months old. Got all my fish then. Put this piece in, as well as a couple other pieces in about 2 months ago. The rock was cured, I got it from a reliable place where I know that it is fully cured.

 

I have had this happen to. I agree with Mr. Bill. Wrong light spectrum or low cal. . The brown stuff is probably diatoms because the rocks you added probably started a mini cycle in you tank. Do a water test if you can and post your params.

 

I too recently added about 40lbs of cured live rock and I'm getting the same results where the coralline algae is turning brown and white. I hope upgrading my lights to MH while using purple up will bring them back to life.