Could you tell us more about why would you like to remove the crank?
I see that you've removed the black cap before, you'll need to put it back on as it will be much harder. You'll need an 8mm hex key to remove it, just hold the other crank to prevent the other side from moving and unscrew the one that you've shown with the key. Just keep in mind that it might be way too difficult to do it by hand and you might need to go to a bike shop where they could do it with an impact wrench.
Hope that helps.