Question
I have a two year old paver walkway and moss is occuring in spots.

What can cause this?

How do you combat it?

Answer
Moss needs constant moisture to grow, so your paver walkway is simply staying wet all the time. I'd guess you have automatic irrigation sprinklers nearby and/or that it's a walkway under trees or in a shaded north-side area.

Decrease the moisture there or use a moss-inhibitor that you can get at Home Depot and similar stores.