Was actually just photographing the plant (Baccharis salicifolia) but noticed later that there were a couple of leaves that were covered heavily in galls. This is the only photo I have. I ran a search on the host in The eriophyid mites of California and came up with the possible candidate of Aceria baccharices. Naturally, there are no images on the internet, but here is an image of similar mulefat galls which the author does identify as Eryophyid mite galls.

Would like to know what the experts think :)