When I saw the title I immediately thought of Genesect but that would be more based on the movie "Predator" than "Alien", wouldn't it? And an "Alien" pokemon based like the one in the movies would destroy every other pokemon as well as its trainer; so it wouldn't be controllable.
has no weaknest an resist everything but eltric types
an only light an eltric type pokemon can learn light moves
Light wouldn't work unless they made it a sub-type of normal (which is possible). This could encompass all those rainbow moves that we already have, all the while give normal pokemon strengths against dark, yet maintain the other properties of a normal type.
In other words, rainbow/light type would be a sub-type of normal and include certain "normal" moves that are strong against dark, yet still retains the normal type properties.
This is the only way I can see light type happening. It's not diverse enough on its own to be a type.