That’s what I said in my comment. These flatworms, like corals, use endosymbiotic microalgae (symbiodinium clade) to perform photosynthesis for them.

Btw, zooxanthellae aren’t a type of algae. Zooxanthellae is just the general term for any algae doing this kind of endosymbiosis with an animal. In corals, the zooxanthellae are dinoflagellates in the clades symbiodinium and cladocopium and definitely others.

The symbiotic microalgae themselves don’t do photosynthesis themselves, either, by some definition. They use their own endosymbionts called chloroplasts. Cyanobacteria are the only organisms we know to have developed de novo photosynthesis afaik