As others have already said, the white stuff is just the magnesium in the anode reacting with the water and sacrificing itself instead of other components, as intended.
The first time I pulled the anode from my water heater, all that was left was a bare wire, about 1/4" in diameter. It came out along with a flood of white crud. It took significant flushing of the system to keep the sink faucet from clogging.
The second replacement was less exciting. After a year in place, the old anode showed only a little pitting, and there was minimal accumulation of crud.