Despite its current tooltip Corrupted Grass can definitely not be found anywhere on the Corruption layer, nor can it be discovered anywhere else on Creativerse-worlds as it is not generated "naturally". Instead Corrupted Grass can only be created by players. For this blocks of ordinary green Grass can be corrupted by throwing Corrupt Bombs at them, or by placing Corrupted Water ledge to ledge with a Grass Block. Corrupted Water will not corrupt blocks diagonally (corner to corner). Neither Dead Grass nor Savannah Grass can be corrupted yet (as of R27 in January 2016).