Big Green stain under my chair mat. What do I do?

There is a large neon green stain (about 2 ft x 2ft) under my chair mat in my bedroom. It wasn't there before putting the chair mat down. I've had the chair mat there for about 7 months and the stain is probably about 1-2 months old. I have not spilled anything on that location. The carpet isn't wet anywhere else other than in that location. As of right now there is 48% indoor humidity in this room. How can I clean this.