he could be rubbing it and what you see the a dragons form of a scab. Just add some neosporin on it and let it heal. If he is rubbing it, more time out of tank and roaming with you may help.
Also there are some mild fungus that appear this color. Use a Q-tip and if you have or can get some nolvasan dip, it onto the area and clean it. You can use peroxide or diluted betadine, but you have to make sure he does not taste it or try toeat it... the nolvasan is safe if he ingest it.
You might also want to check his mouth for any white or whitish yellow, looks like fine cottage cheese in the mouth, if he has that, you will need a vet to see him and treat as that can be either mouthrot or a fungal yeast infection.