Did you cycle the tank and see an ammonia spike, then a nitrite spike, then see both of those things go to zero? Just curious because two weeks is a really quick cycle - unless you used cured live rock.
Regardless of the answer to that... the brown algae is most likely diatoms. Does it kind of look like brown rust and blows around easily? Diatoms are a pretty normal part of cycling/starting up a tank. You could probably put 10-20 snails in there (astrea and trochus are my preference) and they'll make a quick meal of the diatoms.
Diatoms don't mess with coraline growth. Neither do snails. And personally, I'd save the money and stop adding the PurpleUp. If you're doing frequent water changes, have your Calcium above 360 and your alkalinity above 8 dkH
, you'll get coralline... assuming you have some rock in your tank that has it to start with. (You need coraline to make coraline.) If you started with dry base rock (which I hope you didn't - see initial comment about cycling) then no matter what you add you won't see any coraline growth. A good rule to stick with is to test for anything that you dose into your tank. With that said, have you tested your calcium and alkalinity to know where you're at before adding the PurpleUp?
Again... have patience, and you'll see coraline spread. Adding stuff to speed things up does nothing but throw your water parameters off kilter. And in a new tank, it's going to take 3 months or so for things to settle down.