button stuck in background

hi,
I don’t know how this happened, but when I realized it, there’s a button always appearing in every page in my website, like this:


and when i click on the button, this is what would happen:


Can anyone please help me? I’ve been having this problem for a quite a while. What I do now is to change the background color to white in order to cover that button up, but I seriously want to get rid of it.

Thanks in advance!!