Larva appears on underside of maple tree branches. What are they?
That looks like an insect called Cottony Maple Scale (Pulvinaria innumerabilis). They are always around in low numbers in Michigan and do not cause much harm to healthy mature trees. In some years, their populations surge and they become noticeable on twigs and branches. In particular, the honeydew that they produce can be an annoyance to residents because it builds up on surfaces and attracts sugar-loving insects like bees and wasps. We don't recommend treating for them unless the tree is young and/or recently planted. The white fluffy masses that you are seeing now are egg sacs. When they begin to hatch (usually in July), predatory insects like lady beetles will show up and help control the scale population for you. If you'd like to read more about this insect in Michigan you can read this MSU Extension article: https://www.canr.msu.edu/news/lecanium_scale_and_cottony_maple_scale_on_honeylocust_silver_maple_and...
And this article from Ohio State University Extension has more details and photos of the insect: https://ohioline.osu.edu/factsheet/ENT-62