On this page of the Baldur's Gate 3 guide you will find information on whether it is worth giving the witch your eye.
When passing through the southern areas of the map, you can find yourself in the vicinity of the tea house. There you'll meet an old woman - Aunt Ethel - arguing with two brothers. As soon as you get close to them, a conversation starts, after which the woman disappears. If you go deep into the swamp, you'll find her home.
If you decide to confront the woman, she will turn out to be a witch. She'll offer you to remove the parasite in exchange for one eye. If you agree to such a deal, you will still have the parasite, and on top of that, the character talking to Ethel will get a permanent status that reduces her chance of passing the perception test. In addition, the hero will not be able to inflict critical damage. Therefore, this solution is not cost-effective. Your problem will not be solved and one of your team members will be permanently crippled.