Erineum galls are galls caused by a tiny eriophyid mite feeding on the plant tissue. Erineum galls on leaves do not look like what we classically think of as a gall (look at a hackberry leaf for what I consider a typical looking gall), instead erineum galls look like a patch of velvet on the plant leaf. They are usually underneath the leaf and are sometimes reddish in color.
Unfortunately you need a microscope to see the eriophyid mites, but they are really interesting to see. They look a bit like tiny pale carrots crawling around the leaf. Often there will be large numbers of them associated with an erineum gall, all feeding on the plant amongst the velvety looking plant growth.
Eriophyid mites, like galling insects, are able to cause the plant to produce abnormal growth that provides them with a somewhat sheltered place to feed.
There is no need to treat erineum galls (or any leaf gall) the damage to the plant is primarily cosmetic and causes no lasting harm.