The purple coralline algae is often found on Fiji Liverock.IME,this color of coralline doesn't seem to grow on the glass/substrstes like the pink/green coralline does.It will spread to other live rock once you have the purple coralline already.You can chip pieces of the algae off and superglue the pieces to the rock to help establish it.
For some reason,the purple coralline in my tanks seems to be confined to the FIji LR and doesn't spread readily like the pink/other colors do.