I would agree that it is not a Halysidota harrisii tussock moth, which currently is found in eastern North America. I would suggest it is not a caterpillar (i.e., an immature stage of a Lepidoptera), but rather it is a sawfly (a herbivorous type of Hymenoptera). If you found it on or near an alder tree, it is likely a woolly alder sawfly (Eriocampa ovata). The white material is a waxy layer the sawfly secretes around its body rather than long hairs that cover a tussock or tiger moth caterpillar. Eriocampa ovata is not native to North America, it does occur in the Pacific Northwest. Because the wax is secreted, no two individuals look all that similar. At the end of their feeding stage, these sawflies discard the body covering and then pupate.