Yes, that is exactly right. It's not very common, but you do hear about plecos doing it occasionally. It's a rare occurrence, nonetheless. However, that doesn't stop people from shouting that you can't keep plecos with goldfish. That's like telling people not to swim because you might drown.Looks like a Common Plecostomus to me. I've never heard of them doing this but, they are probably eating the slime coating from the goldfish. If so, it is very harmful to the goldfish. Chinese Algae Eaters do this but, as I said, I've never heard of plecos doing it.
This was quite a story - the goldfish survived more than 4 months without a filter after an earthquake.I used to have a goldfish tank, but the fish all died because of filter failure while I stayed the night at a friends house.