I was trying out an adjustment layer and found that it applies to everything under it.
So what if I want to reduce the contrast on just 1 layer at the top of the stack without affecting the others?
Put the Adjustment Layer and the layer you want to adjust into a layer group. Then set the blending mode of the group to "Normal" instead of "Pass Through".
This will isolate the adjustment to that group.
Alternatively place the Adjustment layer just above the layer you want to affect, then place the cursor on the line that divides the two layers in the layers palette: now if you hold down the Alt key your cursor will change to two little circles, click once and the adjustment layer will only affect the layer you want.