Dude, that other fish beat the daylights out of him. The really only thing you can do at this time is keep the water conditions as pristine as possible, this along with as little stress as any rift lake tank can be. Even with all that he still may not make it I'm sorry to say.
With regards to the stringy white feces I would go ahead and treat for digestive issues. I normally recommend feeding a anti-protozoa medicated food but since he's not eating that won't be a viable option. The best medication for this in my opinion is Clout (this will stain silicone for short time). Follow this schedule and remove all carbon from the filtration as this remove and medication.
Day 1. Perform water change then treat at full dosage.
Day 2. Perform a 30% change and re-dose at full strength.
Day 3. Perform a 30% change and re-dose at full strength.
Day 4. Don't do anything again.
Day 5. Perform water change in excess of 50%.
Day 6. Don't do anything again.
Day 7. Massive water change in excess of 60-70%.
Day 8. Feed and see if everyone is eating.
The last thing I would add is Epsom salt dosed slowly at 1 tablespoon per 5 gallons, then replace the only the salt you've removed from the water changes.
The entire tank must be treated as they have been exposed. The last thing don't feed during this time as the digestive track must be clear.
Best option if they're eating.
Medicated Flake Food for Fish
Or if they're spitting or refusing foods.
Amazon.com: Sentry AQ Mardel Clout Tabs, 100ct: Pet Supplies