Unfortunately there doesn't seem to be a built-in way to get this, so some trigonometry is needed. To calculate the direction from sprite 1 to sprite 2 you can calculate the displacement from 1 to 2 in x and y, then use the atan
function to find the required angle:
Since you actually want the direction relative to the direction the alien ship is facing, it might be better to use the vector product (aka cross product):
The screenshots here are taken from this Scratch project.