Certified GKH Gardening Expert
Sometimes the outdoor use potting soils will be less sanitized than some of the other indoor or even indoor & outdoor use potting soils. Remove two to three inches of the potting soil from the pot (depending on the size of the pot), being careful not to disturb the feeder roots. Then replace the removed container soils with some indoor or duel use indoor/outdoor potting soil. That will usually stop the mold and mushroom problems.
One other thing to note though. If your potted rose does not have a drainage hole in the bottom of the pot, the soils are possibly staying too wet due to no drainage, which leads to mold once the soils are saturated and have set for a while. This same thing can happen if the pot is in a tray or saucer at the bottom to catch the water that runs through the soils in the pot and that tray is not drained after 30 to 45 minutes. Saturation of the soils will eventually kill the rosebush as well.