The already mentioned remedies may work. I have done all of them with various degrees of success. The approach that worked best for me and the one I use regularly to clean my mouse fur 'spots' is a product named Folex. It is available from a number of outlets including Home Depot and Lowes.
Folex is an industrial grade carpet spot cleaner that does not use harsh chemicals and is a very effective surfactant to loosen the small, embedded particles. You apply, let sit for a few minutes, and then dab up the remaining moisture with paper towels. Depending on the degree of stain, you may have to assist the chemical using a toothbrush. It does not have an odor nor leaves behind a sticky surface to attract dust.
"Good judgement comes from experience. Experience comes from bad judgement."
Air #48582, S/SO #003, WBCCI #4584