That is a type of hair algae growing on your Marimo there. I get this sometimes too if one decides to float too close to the light source. Just pull off the "hairy" part, and do as other have suggested above and squeeze it out and roll it around a bit. If you can move it to a darker part of the tank that should take care of your problem.
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?