I just added a plec a couple weeks ago in my Oscar tank. There is only ONE. I have these black round spheres on the bottom, I can't explain it any other way. The fish was a trade in at the store, so maybe it was gravid at the time it was traded in?
Those don't look like any fish eggs I have ever seen, look more like amphibian eggs. What kind of plecostomus is it? Anything else in the tank? Supposedly most plecos are difficult to breed in captivity save for ancistrus and their eggs look like this...