Bald patch on otos head!

[reptilenotfish]

I've kept four otos with a male betta in a heavily planted 6.5 gallon (30c) aquarium for a little less than two years now (don't know exactly how long), and haven't had any issues so far. Yesterday, I was watching the tank, and noticed a pale cream bald patch on one of their heads. my first thought was maybe the betta was responsible, but i'm hesitant, as the fish almost never interact and almost always hang out in different areas of the tank, with plenty of plants/hardscape for the fish to feel secure or hide from others. I have only seen him bite once, when I first introduced a nerite to a previous setup he was in, and I haven't seen any other signs of stress from him since. there are some rough edges on the wood I have in there, so that may be the culprit, but if there are any common diseases or other things that you know of, please share what you know . i'll be keeping an eye on the tank and will post if anything changes

note: I can't post a picture of the tank right now, but I have in a previous thread about cleaning moss.