Although this is clearly an early instar, I suspect it may be the Linden prominent moth caterpillar. The species is highly variable, however, Wagner's caterpillar guide says it is highly variable but always recognizable by the low hump over A8 (near the rear) which this one has, as well as two black spots on top of the head, which this also has. Both hump and spots are tiny, but so is this caterpillar which may have only hatched an instar or two ago!