2D Proximity Fade
Fades the layer based on its distance to another 2D layer.
Code
targetLayer = thisComp.layer("Target");
fadeDist = 300;
d = length(position, targetLayer.position);
linear(d, 0, fadeDist, 100, 0)
2Dpositionfadeproximity effectreactive effect
How to Use
Apply this to the Opacity property of your layer. Rename the 'Target' layer or update the expression accordingly. The layer becomes fully visible when close to the target and fades out when it moves farther away.