Unless it's an acrylic grout, the grout itself will be water-soluble. Try soaking it for a little bit. The clean-up may dislodge some of the grout you actually want there so you may need to touch it up afterwards.
Most grout I have ever used dries to something like concrete, partly by design to keep out water when used on a floor on in the shower. Check around your local home improvement store to see if they sell anything to help remove dried grout.