The hairs (curled forwards on one side and rearwards on the other) are typical for the Miller moth Acronicta leporina. Normally the caterpillar is green with white hairs, and very striking. I suspect yours has assumed its dull colouring before leaving its foodplant and wandering in search of a place to pupate, as is often the case with green caterpillars that live in trees. The usual foodplant is Birch or Alder, although they have apparently been recorded from Poplar and Sallow. I suspect there will be a tree of one of these species nearby, most likely Birch.