I have never heard of the “Green Bay octopus.” This is akin to Nessie?
Im pretty well versed on my monsters, especially local ones. There was a Nessie type creature reported further south in Lake Michigan proper during the 1800s, not over in the bay. Having said that, there were many people at that time that started to understand the economic value of such an attraction to an area and suddenly many places around the US were swarmed with all sorts of beasties.
Local natives had a water panther type creature in the lake. For some reason plesiosaurs seemed more likely to settlers than giant tentacle creatures even back then.
The closest octopus monster that Im aware of was down in Devils Lake.
That was supposed to be a giant demonic octopus that appeared there for a couple of summers but was quickly forgotten about.
The closest thing to a sea monster we are proven to get here is rarely a tiny jellyfish that blooms in swarms and then disappears without most locals having ever seen one or believing that they exist.