I personally have never seen black spot that looks like that. But here's something else I'm noticing along with the slight cloudiness - those eyes are bulging! Who else thinks that's a sign of a raging internal bacterial infection? If it's eliminating stringy, white feces, that would be a dead ringer.
The fish should be placed into a hospital tank, ASAP. The salinity should be lowered gradually, at this time I'd hold it to 1.015 - the purpose being relief of osmotic pressure (same as with adding salt for freshies).
The ganty belly looks really bad, too. This is weird because everywhere else the fish appears to be well-fleshed out. Overall, this fish is in bad shape. I would guess that between its skin and other physical features it's in dire need of good nutrition, and QUALITY WATER, as Baio mentioned.
At this point I can't recommend more than a good broad spectrum antibiotic for what I believe may be a secondary bacterial infection - my usual choice is Spectrogram. If you can get more shots, different angles (especially lighting so we can see what's up with the skin) it would be helpful, along with specific water parameters, what was tested for, and what test KITS were used. We'd need at least pH, ammonia, nitrite, nitrate, temperature, and salinity.
Additional occupants, feeding information, tank size, filtration all would further help.