Bus,
It is a little late to be treating it this year as it will die-back through the winter. Chances are, though, that it will be back next year. The probability of having this plant increases greatly with a nutrient-rich pond, but it is not uncommon to find it in ponds that aren't considered "nutrient rich". In fact, I see it often in situations like yours that have lots of tree cover.
If watermeal is indeed what you have, then be prepared for a battle. Forunately, your pond is small so chemical costs shouldn't break you.....they'll just make you mad!
