You have an insect or mite infestation, most likely insects from the
Homoptera
genus. (aphids, scale, mealy bugs.) Sight unseen I'd go for scale
insects which can be smothered with repeat applications of
horticultural oil. ( you might be able to do it with spray vegetable
oils like the brand"Pam")
The first step would be to give the plant a nice warm bath or next
month you will be asking about sooty mold.

Look on the underside of the leaves for little bumps those would be the
covers of scale insects.