Identify a Caterpillar (Millipede?)

Very small - maybe an inch long and a millimeter or so wide - hanging out on a yarrow flower in Southern California.

Brown with yellow stripes (or vice versa), with a very consistent diameter through his whole body. No obvious hair nor horns.

He’s very quick to retreat to hide under the flower.

There’s a webby-looking fortress of solitude that he’s got on top of the flower. (I assume that he hatched out of it.)

What is he? The closest I’ve been able to find is the Trichordestra Legitima but they seem bigger, chubbier, and not generally found on this side of the country.

Sunflower Moth?