I assume these are scale insects. When I saw them I remembered the mysterious sign I spotted earlier in the year in Costa Rica. After seeing this, I think scales were probably responsible for that.
Note how they’re arranged along the central leaf vein. They’re undoubtedly taking plant juices through that midvein.
Click on the photo for a larger version and see if you scan spot a small mite (an “easter egg” I didn’t know about when I took the photo). Then see if you can find its recently cast off skin!