Can a non custodial parent ask for a decrease in child support?

Of course, but there will need to be a good reason for the judge to modify the order. You can ask for anything it just might not happen. We will really need to know more about your situation to give you a better answer and your state too.

Yes, it happens all the time, if things change in the earnings of either or both parents, they can ask for it to be modified

When was the last time your support was adjusted? Also what are the circumstances for a new adjustment ?