Gree & Brown Algae

[tj2004]

I have two different questions. The first is what is causing brown algae on the sand bottom of my tank? I think it might be light. The second is in regards to coraline algae. The back of my tank went from a pink coraline algae to mostly green. I think it is coraline because it is very difficult to scrape off. Is this type common? What would caus the change from pink to green?

Thanks