Yes, that looked like some kind of poop/frass from something. I haven't heard of any butterflies using the amaranths. I'm having trouble with a big outbreak of katydids...eating, eating, eating. Practically defoliated my Lacey Oak and damaged foliage on lots of things. And then there's the Swamp Milkweed Beetle larva eating my milkweed foliage. Killing all of those I find. Here's a Giant Swallowtail, looking good atop a Basketflower (the Basketflowers are all over my yard this year, BTW). I have released two of the GSTs, have two more chrysallises and two cats!