Potatoes are a living organism, but as soon as the tuber (potato) is washed and packed, the clock starts to tick, in regard to their shelf life. After a little time in cool, dark, dry storage, the potato starts to sprout in its quest to take root and reproduce.

So much for the tuber-science. As for your question, yes the potato is still usable for clam chowder (or any preparation method) as long as there remains some reasonable firmness and the potato hasn’t spoiled. Just remove the sprouts and peel and use as usual.