Your expression needs to calculate how long ago the trigger event happened. Also, you didn’t mention how long the fade in should take, so I just set it to .5 seconds:
L = thisComp.layer("Punkt");
M = thisComp.layer("Punkt_Maske");
fadeDur = .5;
if (M.transform.position[0] >= L.transform.position[0]) {
t = time - thisComp.frameDuration;
while (M.transform.position.valueAtTime(t)[0] >= L.transform.position[0]){
t -= thisComp.frameDuration;
}
linear(time-t,0,fadeDur,0,33);
} else {
0;
}