This is on my amaranth.
Please help identify pest, disease
I really can't tell from your pictures but it reminds me of caterpillar feces though more than I have ever seen. LOL . I would try using B.T. It is not a poison to people or other insects. If it isn't caterpillar damage, it won't hurt anything and you can then try a general systemic on it.
"What kills caterpillars?
B.t. to the Rescue Bacillus thuringiensis var. kurstaki is a naturally occurring bacterium that kills certain leaf-eating worms and caterpillars. Simply spray Caterpillar Killer onto trees, shrubs and vegetables so that worms and caterpillars eat the treated vegetation."