This little fellow is (very probably) a Saltmarsh Moth caterpillar (Estigmene acrea), but again this is one of those “identifications” wherein this could also be any one of a number of closely related species.
Below, an earlier instar salt marsh moth caterpillar does an amazing yoga pose:
The just-hatched caterpillars are mostly yellow, with little “tufts” of black spikes; as they grow and molt, their bodies become progressively darker and the “tufts” become a thicker coating of black fuzz. Adult salt marsh moths are white with tiny black spots, somewhat like a Dalmatian.
Saltmarsh caterpillars were originally named after an erroneous belief that they were a pest of salt-grass hay grown near Boston, MA. In fact, although saltmarsh caterpillars do eat a wide variety of plants, they aren’t particularly attracted to grasses, generally preferring broadleaf weeds as younger instars, and moving on to vegetables and field crops as they grow.