You should contact them and offer them to license your code to them and negotiate on a price, since they are already using it. You could also be proactive about

You should contact them and offer them to license your code to them and negotiate on a price, since they are already using it. You could also be proactive about it if you are interested in collaborating with them to continue improving on that library for a salary, since you are the author and the one with the know-how of things. People can be reasonable, try the respectful and honest approach first. They probably missed the fact that the code didn't come with a permissive license and they just added it by mistake. It can happen and doesn't necessarily mean they were being malicious so they'll be willing to make a deal with you.