There are two meshes that are clipping into each other.
I used the boolean modifier to subtract the cylinder from the wall object. How do I fix this problem?
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If you are referring to what can be seen in the image below, this is called z-fighting. It is when to surfaces overlap and are in the same 3d space. This is not physically possible in the real world.
To fix this you would have to get rid of one of the overlapping surfaces.
Select the object
Select the faces that are overlapping.
Comrade's answer is good. If you want to keep both meshes intact, there is an alternative way.
.99to start. Keep adding 9's until you see z-fighting happening again. If so, hit the backspace to erase the last digit.