You have created a displacement on that brush. Simply selecting the brush and pressing the "toW̲orld" button on the right side of Hammer will fix the problem. Alternatively, you can just make a new brush.
[EDIT] You shouldn't use power 4 displacements, they can cause problems with sticky bombs (If you need as much detail as a power 4 displacement split it up into multiple power 2 or 3 displacements). Also, as Idolon said, displacements don't seal maps and you should place a nodraw brush under it.