That kind of bump is hard for me to classify. We're pretty good on naming the issues South American fish have, but I have seen some weird ones on African fish. Since they are nowhere near as popular, we are in the dark about them.
It's probably a bacterial or fungal infection. How vague can I get?
Where it is suggests a bite wound, or a spiking if they were fighting. With good luck, it'll burst and the fish will be fine. With bad luck, it'll spread in a crowded tank. It's hard to guess what it is. Maybe if we could see its texture, we could make a better guess.
Look up columnaris, flexibacter and saprolegia, to see if the images correspond. Don't stop at the first image, as they can take several forms.