Parent a Position X from Particular World to it's own transform xPosition
There's no property called xPosition, and transform is a property of a layer, not a method, but you're on the right track. The expression has to return an array of two values [x, y], and the components of the position are stored in an array like that too. The expression you want — if you're applying it to the layer position property — is this:

xpos=thisComp.layer("Blue").effect("CC Particle World")("Position X");
ypos=transform.position[1];
[xpos, ypos]


That will apply the value of the Position X property of the effect to the X position of the layer, and the current Y value of the layer to the Y.

If you're trying to use the layer's X position on the effect's Position X property then you need to use:

thisComp.layer("Blue").transform.position[0]


you only need one value for this property.


  Pro Tip a great way to learn how properties are accessed in expressions, as well as being a super quick and typo-free way of
  creating them is to use the pickwhip. Next to the expression box
  you'll see a curly icon. Click that and drag it to the property you
  want to reference in your expression. So in the second case you could
  drag it to the number for the X value of the layer's position and it
  would give you the correct syntax for accessing it.  
  
  


A good place to learn more about expressions is Dan Ebbert's page: http://motionscript.com/