My understanding is that’s the level fill sensor. It should as simple as tightening it very gently to stop the leak.
There’s a video on Lelit insider on how to replace the level sensor. Take a look there as it would give you an idea.
If not, then I think you can remove the nut, wrap a few turns of PTFE around and then refit. But it’s best for Dave to confirm. 👍
edit: found the video. PTFE is your friend indeed. 👍