Garden Pests and Diseases: Unidentified pest is attacking all my succulents, 1 by sprucible

sprucible wrote:
In the last two months I've gotten really interested in succulents (with only minimal gardening experience before this) -- I only kind of know what I'm doing. My plants were flourishing here in the San Francisco Bay area, but a week or two ago I noticed some little white/beige spots on my Echeveria Mexican Snowball. They seemed harmless at first, but after a while they expanded into little cancerous-looking marks -- at which point I removed it from the window box in hopes of saving the other plants.

It was too late, though, and over the next several days I noticed it on some echeverias, crassula ovatas, graptoverias, and I think also my crown of thorns plant. I'm trying to figure out how to stop whatever these are, but I can't figure it out.

I don't know anybody who knows anything about garden pests, so I researched online and decided it was probably scale. An employee at my nearby nursery said he thought it was aphid damage, but I can't find any bugs anywhere even though new marks have continued to appear.

So I tried a bunch of things: I got some Bayer "insect protection" spikes and poked them under my plants. I also got some tomato and vegetable insect killer with 0.012 pyrethrin and sprayed it on as thoroughly as possible. (I did it a bit too early on a sunny day and my plants got a little red on the edges, but nothing too serious.) A few days later, with the marks appearing on even more plants, I got some Year-Round spray oil and sprayed that on every surface I could. It must have taken me twelve minutes to feel like I got full coverage.

In related news, I used to see some miscellaneous brownish bugs hopping/flying around on the soil, but I haven't seen any since I initially sprayed the insect killer -- about nine days ago. Despite that, more marks have appeared on more plants -- the echeveria harmsii went from perfect condition to all spotty between last night and this morning.

Does anybody have any ideas what this could be? I would be incredibly appreciative! Sorry for such a long post -- I just wanted to include as much information as might be helpful. The photo of the Mexican Snowball echeveria is probably the best one for seeing the mystery pest.

Thank you!

PS - I found a couple of slugs last month, so I spread some Sluggo around. I have since found two dead slugs, so I don't really think they're likely to be causing this damage.