I recently bought a desert rose (no knowledge of plant beforehand) and it's doing well, but it has brownish yellow scales on its leaves. It takes a bit of scrape to remove a scale 'cause it's dry on the surface but underneath, it's squishy with a yellowish goo. I haven't seen any insects/pests on it and there's no sign of inhabitation whatsoever, so I'm stumped. After I clean the plant, the spots come back a week later. Should I be concerned?