My first thought was an external parasite however, am I mistaken that the bottom end appears to be coming from the anus? If so, it's possibly a dead internal worm making an exit and got hung up on a fin or an active intestinal worm. Many fish straight from the ocean have a white colored feces so the color is not all that concerning......yet.
cardinal fish are schooling fish and if it is alone, the stress of being alone may have something to do with this condition. Has it been through any kind of quarantine? Does it have plenty of places for it to hide in? How's your water chemistry? All these can have a role in your fish's health.