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It's a form of red hair algae and quite nasty stuff. It will spread easily if any parts break free which makes it very important to remove as soon as it's found. It will thow off small pieces of itself that look just like tumbleweeds. Siphon it off very carefully to prevent breakage/spreading and then if possible remove the rock it's on a scrub it clean. Rinse very well and put it back.
It will only get a foothold in area's of low flow so you might want to address that as well. This stuff makes green hair algae look like a cake walk.
The red stuff is fluffy, not fuzzy really. It bums me out to hear that it is a hair algae. Our tank has been doing really good, except for some bubble algae. The bubble doesnt bug me too much.