I got an answer on one of my questions that helped me a lot. However, the answer is incomplete and would fit better as a comment. Although it was very helpful to figure out where the problem was coming from, it doesn't actually provide a solution. The reason why it wasn't posted as a comment is probably because the user who posted it doesn't have enough reputation to do that.
I've already posted a comment on the answer to tell the person who posted it that it was useful but needs more information. If it wasn't useful, I would have flagged it as not an answer, but now I don't want to do that since it would risk getting deleted and deleting this answer would delete valuable information.
What should I do about this? Flag it with a custom flag to ask a moderator to turn it into a comment? Leave it alone? Something else?