Tips of my ground cover roses have a white powder like substance What is it?
How do I treat it?

More than likely it's probably powdery mildew.
Hard to say for sure w/o a photo but that's probably what it is.
Go to HD or Lowes or any good nursery and look for a rose fungicide that will treat Powdery Mildew. Some come already mixed and you just spray them on. Others come as a concentrate and you have to dilute with water, then apply.
If you want to go the organic route, there are concoctions you can make that work just as well using household ingredients.
I have a recipe for one. I'll find it and post it for you.

Here you go:

For 1 Gallon of rose fungicide for Powdery Mildew:
Follow mixing instructions as listed.

Mix the following into ONE CUP of water:
1 1/2 tbs, baking soda
1 tbs. canola oil
1 tbs. Insecticidal Soap ("Safers" is a good brand)
1 tbs. WHITE Vinegar. Add this LAST.

Stir the ingredients well and add water to make 1 gallon of spray.
Apply with a pump sprayer and thoroughly wet the foliage and infected areas.
Apply when the air is calm and not too hot outside.

There are other variations of this recipe, but this is the one I've used for years and it works very well.
If your infestation isn't too severe, you should be able to see the mildew "vanish" as you spray. The spray neutralizes it.
You can reapply the spray about two days later if needed...but mix up a new batch if you do.