What is the difference between????

[fla. keys]

the darker purple coraline algea on the glass of a tank and a lighter more reddish/pinkish color algea growing on my glass?

 

[luke]

Assuming they are both coraline algaes (it sounds like they are), then they are just different species of coraline. There are lots of varieties. Different ones grow in different envirnments.
Luke