It does seem to look like PM. PM is usually caused by certain weather conditions so it may disappear as your spring progresses. It could also be some stress from the change in environment from inside to outside. Just take of any damaged leaves and keep it in good sunlight and adequately watered, no fertilizer, and see if it starts to recover.

Thank you!

Powdery Mildew, you just get some sodium bicarbonate (baking soda) mix pinch of it with a drop of shampoo and spray this mixture on your rose,

Also before doing this, remove all the leaves which are affected with Powdery Mildew.

Keep them in sunlight like Seil advised.

=) Your rose is perfectly fine don't worry, powdery mildew is very common problem with Roses during climate changes.