I removed wallpaper in one of my bedrooms and found a surprise. I don't know what this is exactly so can someone help. What can I do about it?

It looks like the adhesive from the wallpaper. You can spray with water and fabric softner and use a putty knife to remove as much as possible. Clean the walls with TSP and prime with water based kilz-it tinted to your paint color. Might take a couple coats of primer but primer is cheaper than paint.

Dip the tip of your finger in household bleach and touch that dark spot, does it turn verl light brown after a moment?