Huh, they look like planaria to me. They can get to fairly large populations, and most likely came in with the plants you added. The increase in population might be due to the cholla wood (lots of nice food there), but since you put it in boiling water, chances are high that they did not come from the wood itself.
I've heard good things about "No Planaria". But haven't tested it out myself just yet.
So many fish/plants/inverts to keep, not enough aquaria.