A: The growth is lichen which is a combination of algae and fungi which is called symbiosis. The alga provides the food and lichen supplies the protection in the form of a home. Hand pulling the lichen off could be done as the lichen is not truly harming the plant.
Something else must be causing your roses to be under stress. After our discussion we realized the use of heavy river rock around the root area of the roses was most likely the culprit for the rose discomfort. I know of no woody ornamental plant which will do well in Florida’s hot summers with rock mulch around the roots. River rock can be very heavy; it retains heat and has a tendency to compact the soil. I would suggest you remove the rock and replace it with a light soil medium such as compost, peat moss and sand especially around the roses. Top your roses with a couple of inches of pine straw or melaleuca mulch. The rock is not a total loss because you could use it around yucca, aloe or cacti and we have several varieties which do well here.