Worth knowing what you are actually looking for so you can stop worrying or start. Floppy tail is not hanging upside down, it is the tail flopping over his back or off to the side when he rests head-down, instead of hanging along the line of his body.
So go and look at the tail while he is in that position. If it hangs where it should, you have an ordinary gecko on a surface you want to give him alternatives to.