Dave

you could try removing the tile and see how much wetting has happened.....but you might have to re-do some tanking if it damages it...re-tile and try again.....

Or you could leave it to dry and use a grout sealer......

Does the tile it's self have any different feel to the face of it from the others..?..it might well be a faulty tile but it's hard to say by just looking at that photo...