Recently I had this issue on my tank after introducing tang into it. Unfortunately I don't have a big QT to accomodate all my fishes and live rocks (don't have inverts and anemones). So, I decided to treat whole display tank. Based on google search, I found Kordon Ich Attcach is 100% organic and safe on live rocks and corals.
I treated the tank with that for about 11 days and still the tang had the white sopts and rubs against the rocks. Then with the frustration, I treated the tang with Kondon Rid Ich+ which doen't have copper. I was prepared to loose some corals and even may live rocks. I continued the treatment for about 7 days with 3 times water change (20%) in between.
Also when I started Rid Ich+, I had started feeding the fish with garlic soaked food mixed with vitamins. Towards end of 6th day, I introduced cleaner shrimp. It was unbelieveable that tang goes towards cleaner shrimp and give his body for cleaning!. Initially shrimp shy away from tang. Lately, shrimp cleaned tang. Last few days I am seeing tang doesn't have any white spots and doesn't rib against rocks.
In short, I am not sure exactly what helped curing the tang. My feeling is that chemicals may have prevented more hatch, but good nutritious food and frequent water change and the introduction of cleaner shirmp might have played crucial role (nature way!). As long as fish eats well, it is better to feed with good stuff in order to have better immune system.