I have had a 7 inch Plecostomus in a small aquarium with guppies. He has been fed Hikari alage wafers (one/day), all the algae he could find in the tank (not much), and most of a small zucchini every week or two. He did not seem thin, nor did he seem particularly in a hurry when food hit the bottom near him.

Yesterday I moved my goldfish to a larger tank, leaving behind a 18 gallon long tank with plenty of algae--I had let it go a bit anticipating the pleco arrival. I did a big water change, cleaned up the place, but left all of the algae. You could not see through any of the glass!

Pleco had eaten most of a zucchini piece the day before, so I did not expect much action on the algae last night, his first in the new tank. This morning I turn on the lights and the tank is essentially algae-free. He ate it all in one night.

So the question. Is it possible I am way underfeeding him, was he just celebrating his first true meal of algae in a while, or what?

If you had scraped all that algae off and combined it into a ball or something, it probably wouldn't have amounted to much. Not compared to a chunk of zucchini, at least. I wouldn't take it to mean you're underfeeding it, more like there wasnt all that much food matter in there and plecos are efficient at removing it.