Clownfish with white, stringy poop

Hello everyone i recently got a new ocellaris clownfish. Lately, all he does is lie down on the sand and does not move much at all. However, if a shrimp approaches, it will swim away, similarly with my hand/net. He also frequently has a white string hanging from his under side, which I am assuming is poo. After some research, I have found out that this might be an internal parasite. I will attach some pictures of the poo. If it is a parasite, what do I use to treat it?

BIOCUBE 29
BENGAII CARDINAL FISH
OCELLARIS CLOWN
ASSORTED CLEAN UP CREW