best case: just ate something white and is pooping like that. Sometimes clowns poop white, though that's a long string.
worst case: internal parasites/worms
safe option is quarantine + prazipro / metroplex, and in theory you'd need to do that to any fish in there since you'd have to assume all would have it now. However metro/prazi I believe both can be dosed in-tank with caution.
If it persists, it is internal worm or parasite. Keep watching them. It is not immediately deadly typically so should not be an immediate freak out. Could just be a one time thing too. Hopefully that is the case.
To treat, you can mix Metro into their food using Focus. It is a powder that kinda binds the medication. Dosage is a bit hard to determine as they need to eat it, but I can make you a mix I feed in QT. It is suppose to be reef safe but I have only used in a separate tank myself. I rehydrate dried mydis and calanus using medicated water and a vacuum pump so the meds get inside the food. Works the ticket to get the meds in there. My QT tank isn't in use now, and you can borrow it if you think you need to pull them for a water treatment.
Internal Issues For simplicity’s sake, there are 6 basic problems a marine fish may experience on the inside: 1. Intestinal worms Symptoms: White stringy feces, pinched stomach, fish has faded coloration, fish eats voraciously but still seems to be losing weight. Treatment of choice...
I would use Prazi outside your tank personally. Not tried to put it in food myself either. I can make the medicated food if you decide to go that route.
Ok. Its been awhile since I updated. Looks like the partial shred of the RBTA wasn't that bad. The 50% WC cleared the water up. Fish look good although the snowflake had another white string poop. He did host this AM. Yet to see if they will both Host. Coral are happy little algae. Nitrates still running about 10.