I am not sure from your picture that you have aphids here., but the white would be cast skins. Sure sign is honeydew.  However adult aphids are green. The ones that appear to be adults are brownish. Aphids also tend to be under the leaf on rhodies. Bioneem and Insecticidal soap seem effective on most soft-bodied homopterans.
See this URL from Washington
https://hortsense.cahnrs.wsu.edu/fact-sheet/rhododendron-aphids/
Please feel free to contact me again and send a picture of the underside of the leaf.