impossible to remove background when i track on mocha

If you have already removed the background, applied effects, or in any manipulated footage you want to track in Mocha you must pre-compose the layer making sure that you move all attributes to the new composition.

None of your screenshots show both the Switches and Modes column, we can't see if there are effects applied to the problem layer, we don't see any modified properties so we have to assume that you have only used Rotobrush to remove the background from the footage. Next time you post a screenshot please activate the first two icons at the bottom left corner of the timeline so we can see both the switches and modes column and press the U key twice so we can see the modified properties of the layer. Then embed the images by dragging them to the reply field on the forum or use the toolbar. *

As a reference, here is a shot that needs to be pre-composed before using Mocha AE. I've used Extract to remove the sky and moved the 4K shot around in the frame to frame it up.

I'm going to motion stabilize the shoreline in this moving camera shot so that I can insert a digital frog. I'm going to replace the sky. After the shot is tracked I'll open the pre-comp and turn off the simple mask. I created the mask instead of using a track matte in Mocha because it was very quick and very easy to do with a mask. 

* The "Drag & Drop here..." area is buggy and should not be used to share images.  It's slow, does not work reliably on a slow connection, and it's easier to see what is going on with your footage if it just shows up in your post.