These are spongy moth caterpillars (formerly gypsy moth) that have fallen down from trees. Unfortunately, there is not much to do about them right now. They will feed on leaves for the next 2 -3 weeks and then pupate and become adults. 

If you have a tree or two that is infested, you can set up sticky bands or burlap wraps to catch, collect, and kill the caterpillars as they climb up the tree.

In August, search the trunks for signs of spongy moth egg masses and scrape any off into soapy water to stop them from emerging next season.
See here for additional information: https://fyi.extension.wisc.edu/spongymothinwisconsin/pest-management-2/management-guide-for-homeowners/