I believe it is mostly calcium with some other trace minerals. In short, it isn't like it is coralline algae that will latch on to your rocks and start growing. You'll still need to seed the tank with some.
So, in short, you're better off just monitoring your Ca, Alk, and Mg and doing regular water changes... patience, I suppose, is a virtue with coralline.
This does highlight an important issue in reefkeeping though (and perhaps in everything). I don't want to say a lot of products rely on false advertising to sell themselves to newbies, but that's essentially what it is. If you're armed with just one piece of information (that coralline algae is good algae compared with the other types), you'll see purple up and think you need it. If you already know about monitoring Ca or just follow the instructions on your Ca supplement, you tank will quickly be overdosed with Ca. Purple Up is not the only offender...
There are a ton of treatments and supplements that do not clearly state exactly what they do (or have in them) in terms that anyone can understand... especially new people. If you don't believe me, check out the labels on the bottles on the shelves of the LFS's.
I can only assume that this situation helps sell products (at least a few) to each new person that comes into the hobby and fails or gets fed up because of the millions of supplements someone told them they needed and that don't seem to be helping. It surely doesn't benefit the hobby by getting and retaining people.
Sure, it is on the hobbyist to educate themselves, but most people don't have the patience to spend six months learning everything they need to before even starting to collect equipment and supplies.