Sorry for the late reply.
I am not really knowledgeable on fish diseases as I usually only research particular diseases I have encountered. But if I had to take my best guess as to what it looks like that serape tetra has, I think it's Fin and Tail Rot (although not rotting fins, the tail itself has rot and they body does appear to have white film over it). It could be an ulcer, but fin rot uses same medication. If it is fin rot, I know there are plant and/or invert safe meds out there.
I believe there is a salt and heat treatment as well, but I personally don't like those methods. I have tried with ich outbreak before and wasn't pleased with it (plants didn't like it at all, fish either and I still needed to treat with meds). But it has worked for others and might work for you.
There are also wide-spectrum meds out there that treat for multiple diseases in case you are not 100% sure it has more chance of working in case it is something else the meds does treat for (for instance, Kordon Ich Attack and Kordon Rid Fungus, said to be same exact ingredients and are all natural, fish, plant and invert safe do treat for multiple things). Even Melafix and Pimafix are said to be safe for plants and inverts (although these two meds effectiveness are a toss up if they actually work or not, some say they heal, but do not cure, some say they work, some say they don't)
I would treat ASAP, since the rot exposes raw flesh which exposes the fish to even more bacterial and fungal infections which can overcome the fish rather quickly.
Again I am not 100% sure this is what the serape has, but it's my best guess.
Research symptoms to be more certain.
Possibly could be columnaris, but I think that usually shows more signs of white patches and fungus (but I assume the meds would be the same to treat fin rot and columnaris)
Also I cannot say whether or not this disease is what is killing your Celebes Rainbowfish.
Posting the link to your other thread so other can check it out and give their opinion: