First, try washing in extremely hot water. If that doesn't work, then
I recommend Rit Brand Color Remover, probably three or four boxes of it. Read the label to make sure you buy enough, and follow the instructions closely. It is sold with Rit All Purpose dye in most grocery stores and pharmacies in the US. 

I do not much like all-purpose dye, which is the type of dye the Rit Dye Company sells, but Rit Color Remover is great stuff. See my page, "What chemicals can be used to remove dye?"

It is possible that the Rit Color Remover will also remove the original gray color of your sheets, but this will probably be preferable to the messy green splotches.