I find that my corraline is a good indicator that my calcium level may be low. When I fail to test every week, like I should, and my corraline takes on a different color I instantly start testing. Boy, they sure don't give you a break!!I have alot of coral in my tank which uses up a lot of the calcium, then add the macro algae, I find that I am constantly testing for something.
As far as scrubbing your rock, I too did that when the red slime was out of control, but I have found that if you keep the calcium level up,as well as the alkalinity, the corraline will regrow quickly.