I'm not speaking specifically to Alexander in this post, but just in athletes in general on Twitter or social media. They don't seem to be the smartest bunch at realizing how things will come off to others very well. That or they just say something and lie that wasn't what they meant and/or like WR Adams just enjoy trolling the internet.
Like in theory, Alexander could have just been thanking Packers fans for the last 6 years, and hopes it continues another 6 years, but only wrote the first part, and it's coming off differently now. Not saying that's what he's meant, but other players have supposedly done similar things like that in the past.
That, or if he's truly unhappy, maybe requesting a trade, as he has talked about being tired of getting in trouble for talking to the media. Which seemed to be things like the defense can't let the other team score a single TD with how the offense is playing right now. And then whatever happened with the Panthers situation.