Green Point Lighthouse, the oldest lighthouse in South Africa (1824), is situated in the Cape Town Atlantic Seaboard suburb of Mouille Point, not in the suburb of Green Point, which is a few hundred metres away.
Feel free to take a moment to let that sink in. Your brief confusion is perfectly natural.
The Green Point Lighthouse is actually so easy to find in Mouille Point that I don't even need to post a photo of it (and, in any case, LEGO totally trumps everything else in my world).
You can find this little guy just inside the entrance to the lighthouse near the reception desk. He's been carefully placed behind the photo's decorative porthole to keep him nice and safe.
Entrance to the lighthouse costs R15 and it's open from Monday to Friday, 10am to 3pm.