"I am the Law"
- Judge Dredd (try not to say it like Stallone did though.)
The Greys do not have willpower or humanity or anything along these lines. Instead they have Belief. This begins at ten points. It represents their core belief that science will explain all. It's the belief that the ship will be back for them or the belief that death is near or that tomorrow the sun will rise again. It can calm them in times of trouble. Again, being alien minds it's up to the GM to consider what qualifies as trouble. Each time one encounters something that can not be explained not matter what, they lose a point of belief. If they should ever be allowed to drop below five they begin doubting themselves. Should it fall below one they doubt everything. This doubt creates pure chaos within the mind of alien and (due to their powerful minds) in their immediate vicinity. It is due to people trying to structure the aliens reality and enforcing their beliefs on it that produces this backlash of chaos against the order set into it. In effect, an alien marauder will have been created, insane and bending reality to it's will with no paradox, no regard and no human ideals of how reality should be. (Play this up as a bad thing to your players. Think Akira.)
Other rules are affected as well. Any mind reading attempts will result in a intelligence/wits roll on 9. Derangements for botched attempts. Illusions do work but as said, it depends on how and when it's used (aliens do not confuse so easily. A "wall" that is not meant to be there, they will see right through, while an illusion of a floating cup will be believed, albeit explained rationally ).
(The following are included as fillers and at request from people who wanted to see some sort of points system in the game. As these have been designed to be NPC's it seems pointless to have dots to decide if they can or they can't, but there is no rationalising with some people. Powers will not be changed however)
Co-Habitation - Subterfuge based. Being able to hide amongst humanity for extended periods of time and to pass for one of them. Used to see if they make the right choice in any day to day activity.
00........"Eugh. How can you primates do that?"
000......"Will you stop calling me High Commander in public?"
0000...."Just a glass of dihydrogen-monoxide thanks."
00000.."Honey? Can you get me my pipe?"
Abduction - Comes off stealth. The ability to bring an alien craft onto the planet and grab someone/mutilate a cow/wear silly antennas on their heads and make "beep beep" noises in front of some poor schmuck who no one is going to believe and leave with out detection. Used to see if they make a wrong move when stealing people.
0.........."We're standing here at the Roswell crash site...."
00........*what does electric eye mean?*
000......"Paw? Them lights is back again....."
0000...."What? Where? Well, I can't see it. Stop joking will you."
00000.."Woomera and Jodrel Bank looked right through them."
Cloning - pretty important one to have really. Very few Alphans have it. This comes off medicine and also includes the ability to alter the genetic structure to whatever ends the tester wishes. Used to see the results of any cloning experiment.
0.........."How did that fly get in there?"
00........"Um... Are they meant to have five limbs?"
0000...."Kinda looks like Elvis."
00000.."Mr. President, meet Mr. President."
As for the rest of it do what suits the feel of you're campaign.