I don't think German Smear was ever intended to be applied to the floor. It's a brick wall finish. I'm guessing the white area is where your mortar is thick and the gray area is where the color of the brick shows through.

You know how dirty grout gets between tiles? That's because it's porous, and now you have spread that over your floor so it is going to get dirty in no time at all, and it's likely it will never come clean.

I believe there are brick tiles that have a white washed appearance that is fired onto the brick when it manufactured. That might have been a better way to go.

From Floor and Decor

Belt sander with (lots of) 50 grit?

I don't have any suggestions for how to change it, but I think it looks kinda cool, like there is weathered variation to it.

Murud acid and wire brush will clean up the brick n leave u with the aged look mortar wash your looking for. Then u need to add a sealant to keep clean. Some time n work it will get fixed. I’ve used method to fix a my first mortar mistake. Shield your eyes m wear chemical gloves. The other poster suggests sander I didn’t try it cause I was doing a small fireplace hearth. But with such a large area it may work better for ya good luck

I'd try a wire brush on a buffer but that's because I own buffers and have a wire brush. Perhaps a 7" grinder with multiple soft wire brushes would work.