what is this white square in center of my frame

You have shown the Effects Control Panel, but if you select the layer and press the U key twice you'll see everything that you changed. All the Lumetry changes will show up as well as changes to  Remove Grain. If Remove Grain does not show up in the timeline you didn't make any changes so pressing "e" will show you all effects added to your layer.

Here's a workflow hint for you. If you are going to use Warp Stabilizer on some footage it is usually more efficient to run it then pre-compose the layer and add your effects to the pre-comp. This eliminates the chance of having WS run again while it is rendering. An even better workflow, especially if you are doing some compositing that involves adding things to a scene using tracking tools, is to insert the objects into the scene, finalize the comp, then pre-compose the entire thing and run Warp Stabilizer on the whole comp. This will put the same distortions on the added objects that are on the original shot and make the composite better. Motion tracking will also work better on the original footage and if you run AE's camera tracker it will always work better on the original footage.

One more suggestion. If the shot you need to Warp Stabilize in AE is longer than a few seconds and/or you are adding a lot of effects, it is always a very good idea to render the comp to a suitable production format. Warp Stabilizer bloats the AEP file size, can cause rendering problems because it eats up a lot of system resources, and when you get back to Premiere Pro and replace the original shot with the rendered FX shot, you won't have any problems with the previews. In my production pipeline I use Dynamic Link all the time to start the AE comp, then Undo It in PPro immediately, render the FX shot, and put the FX shot in the Sequence just above the original shot. I only use Dynamic Link as it was designed for shots that are extremely simple and that will render at about 4 or 5 frames per second.