That looks like a bolt!Is there a nut at the back?You may need to hold nut at the same time as you undo the front bolt.
Best thing is to spray some wd40 there ,or a Wurth "rust off ice "spray will ice down bolt to -40º.Then try and undo it.
If still doesnt budge,then only way is is to use air line tool wrench to hammer impact the bolt.