My battery is at 12 V but the car is not starting (dashboard lights do come on but car doesn't start) and I noticed this junk around the positive terminal. How do I clean it?
Looking at the photo it appears as if there is a gap between the battery post and the terminal. If I'm right about that, part of your problem is that the terminal (the part that clamps around the post) may be upside down. The post is tapered and the terminal is tapered to match it. If the terminal is upside down there will be only a small ring of contact between the terminal and the post. Among other problems this can lead to a high resistance connection that could cause your symptoms.
Now, for cleaning – use a stiff brush and some water to remove the worst of the mess. Usually it comes off pretty easily. Then pull off the battery terminals and use a brush like this
to clean the posts and and the inside of the terminal. While you've got the terminal off check its condition, if there is obvious corrosion damage to the terminal or the cable it would be a good idea to replace them.
Before putting everything back together finish the cleaning job using plenty of water. There may be battery acid in the junk on the top so wearing gloves and keeping it off of your clothes and skin is a good idea. When you reassemble a light coating of grease will help to prevent corrosion. Double check that both terminals are installed right side up and that they seat firmly on the posts all the way up.