Small, soft and green, clustered on a shoot tip, points first to an aphid. A few are nothing to worry about: UC IPM says most established plants tolerate aphid feeding and outgrow the damage. But in warm weather a newborn aphid can be breeding within seven to eight days, so a colony can build fast, and it is worth looking again within the week. Before reaching for anything, check for aphids that are already dead: crusty, puffy and golden brown, stuck to the leaf. Those are parasitised mummies, and UC IPM says that once they appear the aphid population is likely to drop substantially within a week or two.
The rest of what gets called “tiny green bugs on plants”. Some are pests we have not profiled yet; some are insects doing you a favour; some are not insects at all, and no treatment will help.
Far bigger than anything anyone would call tiny, growing to about 1½ inches, but people searching for green bugs find them. They chew ragged holes; aphids suck sap through needle-like mouthparts and never chew holes.
Cabbage looper →Legless, slug-like maggots that taper towards the head, often greenish but also brown, pink or whitish, moving through an aphid colony. They look like a pest and are eating one: UC IPM puts it at about 100 to 400 aphids per larva before it pupates.
Hoverflies →Soft-bodied and usually green, with golden eyes and clear, veined wings held like a roof over the body. The adults feed on nectar and honeydew; it is their larvae that eat aphids.
Green lacewing →Aphids turned crusty, puffy and brown, gold or tan, stuck fast to the leaf. Each holds a developing parasitic wasp; a neat round hole means the adult wasp has already emerged. Leave them, and avoid broad-spectrum, persistent insecticides, which UC IPM says are toxic to these natural enemies.
Aphid parasitoid wasp →Four questions about the damage separate these better than an appearance can, and this page answers some of them for you.