new to this. how do i get the purple alge on rocks

it will grow but it takes time. Make sure your prams are maintained and if you have some other live rock in there it will help kick start it.
 
prams = Calcium and Alkalinity

Here's a good read for you on all the parameters:
http://reefkeeping.com/issues/2004-05/rhf/index.php
 
I had a friend scrape the Coraline off the back of his glass and i came home and dumped it into my tank. Seems like 4 days later i could see pink/purple spots everywhere. Then it just took off.
 
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