What gets rid of the small, whitish, hard lumps on the leaves and stems of my houseplants?

The lumps are probably scale insects. They are often white but may be gray or brown. When the insects are young and actively moving around (which isn’t very often), they can be controlled with malathion spray, applied two or three times at weekly intervals. But once they mature, settle down on a leaf or stem, and develop their hard, scaly armor, such treatment is ineffective. The best thing is to rub them off carefully with a cloth soaked in soapy water.