This could be a combination of aggression and water quality.
Are you feeding the fry bbs? The spots could be bbs being caught in the slime coat. Maybe gently rub an area and see it it comes off???
Does your water contain chloramines? If so, RO won't remove all chloramines so a water conditioner like Prime should be used.
I would get them on regular tap quickly. RO doesn't contain necessary minerals that discus require to grow healthy. RO and low TDS are only required at the time of spawning. Also, RO Ph can fluctuate easily since there are no buffers, even at 50% changed daily.