I have no experienced with this L breed, but my leopard sailfin pleco loves zucchini and hikari algae wafers... googling someone on another forum that does own this l breed says zucchini is thier's fav as well, see here
Peronsally I'd contact the person/company you bought them from and ask what they feed. If they say "they live off algae on the tank" or "they eat fish flakes like everything else" expect it to probably be very starved, sick, and likely to die (hope you don't get that answer).
That said I'd try small portions of different veggies for them, always feed in the same general area of the tank so once they do figure out tis food they know "i find food here" it may take a few tried to get them to eat something, but always put in "fresh" food don't let stuff rot in the tank a few days it will jack up nitrates.
Though mine is particular to zuchinii he's also eaten: butternut squash, Mellon (some yellow one and water melon.. these should be consider these and other fruits like sugary treats, don't feed them too often), brocholii, lettuce (romane.. iceberg is nutritionally useless), spinach, brucel sprouts (fouls water so do a w/c next day), cucumber, peas (good if he's constipated or has swim bladder issues), what else... he's tried carrots but not been fond of them or 'regular' yellow squash. You can try papaya, I think its native to some regions plecos live (not sure if it grows near rivers yours would have lived in).
Again mine is a different breed, and every pleco has different favorites.