pink spots on tank walls

Question
I have a 4o gallon tank that has been set up for one year.  In the past month I have noticed pink spots forming on the walls of the acrylic tank.  They hard hard to remove.  Previous to that, I had red algae problem which I treated with API Marine Algae Fix......within a short time that problem cleared up.  It looks like the tank has measels....

Answer
Hey Laura

Its coraline, its not a pest algae. its desirable because of how beautiful it is. really, theres no way to stop coraline, its just a sign of a healthy tank. some urchins will eat it, but otherwise if you dont want coraline all you can do is scrape it