Can you get a more in focus photo of the oto? from looking at the underside of mine on the glass I don't see anything abnormal about yours. The small light spot under the mouth I believe is part of their organs .
It does look like an internal organ, but the other 6 don't have it.
Here's a group picture for comparison. The Oto in question is the 3rd from the left.
(I'm a bit camera challenged and this is the sharpest I could get).