I was wondering, alien planets would be the ideal thing to terraform so can you terraform them? I'm not sure where i read it but I thought somewhere it said that you could only terraform water and food planets.

You are correct, you may only terraform water and grain planets.

The only thing a terraformer does is score you 4 points when you build it, so I'm not sure why terraforming an alien world would be better than terraforming any other.

Because you can't build anything else on an alien planet unless you want points, so you might as well just do that. That would be a very interesting game though, just building a factory on a water and a grain source, and then terraform as many planets as you can, and then play the score card. Which would give you 3 points for  every terraformer you have, and 3 points for being in the lead which you probably will be. And then if you wanted to build another colony you could always trade a few water or grains. I think I'll try it out next time I play, and then I'll post for you to see how it went