I've had that algae before, it starts to plate outwards after a while. Its actually kinda cool looking when it takes off but it can get pretty invasive in the tank. When it plates you can tear it out but the base attached to the rock will stay. I bought a Naso tang and it LOVED IT. Rid the tank of every speck and then found out later that many others had the same experience with a Naso Tang. So if it does get bad and your tank is big enough that is an option. Plus who doesn't love naso tangs right?