Just clean it off or trim feathers if it is too cold to wash their vents. The poo could have irritated them or flies can bite them. There is always a danger of them getting fly strike and having maggots infest the vent, so it's a good idea to keep them fairly cleaned off. Some chickens just have too much fuzz for the poo to fall off, and I trim a few of my hens occasionally. Vent gleet may also be a problem if there is redness with patches, oozing liquid poo, and a bad smell. This is a fungal infection that can be treated with Nustock cream or Miconozole to the cleaned vent, and probiotics orally.
Of course if you meant red poo, as in bloody, I would treat for coccidiosis.