My guess because they are following the veins of the plant they could be scale nymphs.
The other thing it could be is white flies. White fly eggs are usually in spiral patterns and the adult white fly will be flitting around the plant especially when you shake the leaves. They are usually more dispersed on the leaf.
In either case hose off the leaves with a jet of water and then spray the leaves and stems with a horticultural oil. The oil smothers them. Check nearby plants, they usually travel from plant to plant. You will need to repeat treatment every 3-5 days until they are gone. Scale nymphs fall off but once the scales have hardened they need to be scraped off.