These are mealy bug. One of few insects, thankfully, that actually feed on palms and cycads.
They feed by sucking out the plant juices from the plant. Like the aphid, mealy bugs excrete a honeydew substance over plant surfaces. Sometimes a secondary fungus called black sooty mould grows on the honeydew causing the plant foliage to look like it is covered with chimney soot.
How to control them?
If you have a mild infestation you can use isopropyl alcohol to control them. Dab this undiluted directly onto the insect using a cotton ball. Re-apply at 6 day intervals. If you have a severe infestation you may need to spray the plant with a chemical insecticide like Confidor.