Then it sounds like red slime algae. There really isn't anything that will eat it, but it usually results from poor water quality (like other nuisance algaes) and poor flow. Usually aiming a powerhead at the area will help. Red slime outbreaks are totally normal in new tanks. I am going through the tail end of one myself right now in my 90 gallon. Just keep up with your water changes and suck as much of it out with a turkey baster as you can. It will get better!
On a 125 gallon tank, you should do 10% once a week, or 20% every other week. So you're doing okay -- 10 gallons is right around 10%. You should do them more frequently if you are having algae problems.
Is there stringy stuff coming off of it, with bubbles at the ends of the strings?