If it rubs off easily onto a paper towel, then it is likely rust. Rust is a pretty benign infection, if you have KBG, you will get it at some point. Generally it's recommended to fertilize so the grass will grow a bit faster, and then you can mow it off, but be sure to catch the clippings and dispose of them. Do a google search on "Rust on my grass" and you will get many good sites, usually universities, with info on this turf disease.