ID Rock Growth

[rferraro21]

Bought this tank, my first, used. Tank was drained and the one rock it came with was dry for 2 to 3 days. Tank has been up and running for about 7 weeks now. The rock has turned a nice purple with some white spongy? growth on it. Wondering what it is, and is it normal/good? Thanks

 

[Kdean71]

I have some rock doing same thing.. been wondering too