Hm, it kind of looks like mechanical damage to me - did you have a hail storm recently? The newer leaves look like they're not affected so much.
Make sure the damage is still going on before you spray anything on milkweed. What you spray may be toxic to the butterflies - you did plant it for butterflies, right?
Also, once the butterflies find it, you realize the caterpillars will absolutely eat all the leaves anyway? They use it for nectar and also as a host plant for the caterpillars. It doesn't hurt the plant, because the leaves grow but it will be just naked sticks for a while. I like to plant something else around it to hide the sticks, like Gaillardia or Salvia.
