That is surface rust.
You can remove the bolt and clean it off using sandpaper and then a rust fixer like "rustbuster" and then paint it to keep the rust away. I find grey primer spray paint works well enough once hardened. Never had much luck with chrome-paints though.
You should apply some penetrating oil before undoing the bolt, and make sure your hex tool is well-seated in the bolt before applying leverage.
There are three bolts pictured
the smaller one at the top of the photo is part of the lock-on clamp holding the grip in place. It can be removed, there is likely no nut because it threads into the clamp. There will be a matching bolt at the outboard side of the grip too.
The big bolt closest to the middle of the photo is probably holding your brake lever to the bars. It can be undone completely. There is a medium chance of having a nut on the other side, so don't loose it.
The bolt at the bottom of the photo will be holding your gear levers in place. Same as for the brake lever, this can be undone completely and some chance of a nut.
Without these bolts in place you should not ride the bike until they are replaced. Your brake lever and gear pod will slide around the bars and would be unsafe to ride. If you need to ride your bike to the shop, take some tools and undo the bolt once you get there. Fit the new or refit the old one before you ride home.
Be mindful of the angles when you refit a bolt, and make sure the tool is well-seated in the socket before applying torque.