Rock changing color

[rara12]

I set up my ten gallon mini reef about a month ago. After setting up the rocks and adding a few corals the rock starting growing the caroline algea, as expected. But then one of the rocks turned yellow, neon yellow. Does anyone know what this is?

 

[anonome]

Any chance of a photo? There are sponges that are very bright yellow. I have a rock that is covered with yellow sponges. Just a thought.