I'd say it's either a runt and it will be just fine OR the oral decided to grow in a different manner and will just cover it up and let it die off. Whenever I move my larger duncan colonies, there are always heads that just fade away and die off. They like to grow in a particular manner and when the angle or current is changed, they'll adapt and let some heads die. That's part of the reason why it's so hard for me to frag my colony; I've moved it a few times and there's no rhyme or reason to the growth.