Looks like scale infestation - but if, when you get them off, they crumble into dust, they may already be dead. Clean off the scales with a disposable cloth lightly moistened in methylated spirits, without getting it on the leaves. Check the backs of leaves to make sure there aren't any nymphs (immature scales) present - these can look yellow, white, brown, but are usually clustered around the central rib below the leaf. If you find nymphs, you will need to use a pesticide spray which treats for scale.