I'm not sure, but it kind of looks like mold, but it could be insects. It would help if I could see the whole plant. That leaf looks wide for butterflyweed. Asclepias tuberosa likes it more on the dry side. I'd like to see a picture of the entire plant to see if it really is tuberosa, and it might help clarify what's wrong.

Your plant is Asclepias curassavica, Tropical Milkweed. Asclepias tuberosa, Butterfly Weed, has narrower leaves and plain orange flowers. The white fuzz is probably a mold that grew in moist conditions, but it's probably nothing serious. I bought Asclepias viridis tubers from HD, and they also were not viridis, but syriaca.

Hmm, sounds like a problem with their plant vendors, then.

I've seen several tiny caterpillars over the last few months, but they all disappeared before cocooning. I was pretty sad.

You must be further south, because the monarchs only recently started laying eggs here in Illinois. It is sad when cats disappear. I plant a lot of different milkweeds and hope a good number will survive. I tried raising them in cages and it was overwhelming.

Oh, right, I live in southern California.

It was so weird, I found a big one in early June, almost an inch long. Then he just disappeared. I've seen at least 6-7 tiny ones on the leaves, and then they disappeared too.

I think it could have been some wasps that started showing up around the same time, maybe eating or flying off with them.

Charles, a few members in that group also live in southern California.

Here is my so-far happy story: Last April, while walking down the road, I found more Swamp Milkweed seeds in their fluff than you can imagine. Collected some, seeded them in my vegetable garden in May, and lo and behold, it is now July, they're 6 inches tall and have teeny little yellow & black caterpillars. Never seen such tiny ones. FWIW....