Floating chat(?) button suddenly appeared.

[Elytherial]

I know i must have clicked something I tried uninstalling the most recent apps that I installed, too. S8+ on Verizon.

When I press it, it vibrates, but that's all, as far as I can tell. Also the black part will turn blue.

 

[B. Diddy]

It's an Accessibility option called Select to Speak:

https://www.youtube.com/watch?v=NBZZMveGUms