Simple permanent marker to mark it in the future. Will rub off in time. To clean fully some camera lens cleaners with Isopropyl Alcohol in. Be careful of the acetone; it can damage the fabric over time. As someone mentioned - wash it afterwards.
You want to use fine permanent marker pens to write with (no of these super size board markers) and get yourself down to boots and buy some nail varnish removal pads they work awesome in the field. Don't buy a mahoosive fcuk off bottle of the stuff just get a small tub of the pads. unless you want it for back in barracks then get a rag and a large bottle.