I would suggest live food to get it eating...live blackworms if you can find it is best for these omnivore plecos. Second thing is you need to deworm wild caught plecos during quarantine.
Try bloodworms and NLS Thera AA it has garlic in it. I would tyr this a few days then as was said treat for parasites since this fish is probally wild caught. Keep us posted. I have a large 14 inch female L25.