What are these white spots on a dracena
These are plant pathogens commonly known as scale insect (Coccoidea). Among the genera mentioned to infect Dracaena marginata here and here the most likely candidate is Pinnaspis.

They are close relatives of aphids and psyllids (and more distantly -- also to true bugs). Cochineal is probably the most famous representative of the superfamily. Scale insects suck host's sap and can be quite dangerous to the plant. Adult females are immobile and the spreading is performed by nymphs and (when present) males.

As far as I know they are harmless to humans. When digested (products of) at least cochineal can cause allergic reaction in some persons.