That square thing is the drive end of the speedometer cable. The bearing grease cap will just pull off over it.
The cap itself is a light press fit, so you need to tap it along the inner rim to get it off. (There are special tools made to do it, but I've never bought one.)
BTW, when putting it together, you can pull the cable end out a little to line up with the square, then tap the cap on gently.