[Auballagh]

That looks a lot like LYMPHOCYSTIS. Which despite the somewhat ominous sounding name, is basically a skin wart caused by a virus.

Albino Oscar with bump on chin

My albino oscar in the last week has developed a bump on his chin and I'm not sure if it's an illness or from rubbing up against the glass or bumping into things. He is behaving normally and does eat still. I have another oscar and other fish in the tank and none show signs of similiar bumps...

www.cichlid-forum.com

And, just like in us when we (as humans) get these things, there is not a whole lot you can do for them to treat or remove them. With time and some patience, those growths will eventually fade out and disappear.

 

[Auballagh]

Oh wow....
Beware the dead creature problem in an aquarium! Yikes! With a closed little box like that, there's just nowhere for those dissolved organics to go.
So yes, the lack of food interest thing, could definitely be related to that Nitrate explosion (and runaway, potentially harmful bacteria bloom as well) in your tank. I would certainly keep up those water changes until you get the Nitrates in there back down to very low levels. If you are doing the water change thing as remediation for potential problems, suspected illness, etc... then it is recommended that you push higher percentage and frequency water changes to get that measured Nitrate level down in your water as close as possible, or very close to zero.
As for the visible growth on your Severum? It's fortunate that growth doesn't look anything like the pitting and lesions associated with a HLLE/HITH outbreak.

 

[Auballagh]

I'm as mystified by the condition of your Severum as you are. Not sure, but the bumps on his head may be simply coincidental to the problem you are having in getting him to eat. In an Oscar I rescued with a really bad case of HITH (the only Oscar I've ever had actually), that fish initially looked physically terrible with lesions and pits all over his head area. But, that Cichlid still had quite the appetite and otherwise seemed 'normal' in its behavior and other aspects.
So yes, if you are out of ideas, you could definitely move the Severum into quarantine. I would initially treat this fish with Epsom salts in a dip to see if there is some kind of intestinal blockage or something going on that would keep it from eating. As per the handy google search....
To give your fish an Epsom salt bath, pour half of the tank's water into a clean container. Add 1 tablespoon of Epsom salt for every 1 gallon of water. Have the fish swim in the solution for 15 to 30 minutes. Remove the fish promptly and return him to his aquarium if he appears stressed or relieves himself.
If you can get him eating again, I would try to offer some shelled, fresh peas first. The veggie content will be good for that Severum, plus those things move pretty easily through a fish's intestinal tract. Be careful using medication(s) or other things to treat him with. A LOT of fish are lost due to the indiscriminate/overuse of medication, than almost any other thing. And, IF you do place this Severum in quarantine, you are signing up to do DAILY water changes for that fish. That alone may provide the best therapy you can offer him. Be sure the quarantine tank is sited in a quiet area, and resist the urge to keep it lighted so you can see and continually check on him. A quiet, dark and secure environment is what he needs now - with plenty of clean, fresh water pushed through his tank!

 

[Auballagh]

Sure! Frozen might not QUITE be as nice as fresh ones. But after peeling the skin off, the frozen ones should work out just fine. Hope you can get him to eat.