Looking to me like a Yorkshire Terrier dressed for Halloween, this is actually a Milkweed Tussock Moth caterpillar (Euchaetes egle), out and about in late summer and now, early autumn. These larvae principally choose Milkweed (Asclepias) or Dogbane (Apocynum) as their host plants.
Not to be of concern though. These caterpillars generally go for older plants which are more tough, for their meals. Monarch caterpillars will choose more tender, younger plants to sustain themselves.
The Milkweed Tussock Moth caterpillars might leave the Milkweed plants looking a bit bedraggled but those plants will survive to sprout up again next year, for another year of supporting Monarchs and these Milkweed Tussock Moth caterpillars.
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