I think as others have said above, it looks like the water is dropping down from above.
Is that black wire acting as a siphon?
Anyway, if you do have a leak in the lower blue pipe and want to seal it, I think you might be able to use Aqua Knead It, which you can get from Bunnings.
It says it can't be used on
Polyethylene, but it doesn't say it can't be used on PVC, which is what that blue pipe is made from.
I would roll a bit of Aqua Knead It putty and squash it onto the spot and see if that works. Hold it there with some tape for maybe half an hour. After that it will be rock hard. You can chip it off if you need to remove it later.
Firstly though, do as others have said above, stop the flow, take off that piece of pipe and clean the inside areas where the pipes stick together as all sort of crap might be in there causing it not to seal and thus leak. Probably home to bristleworms. If it is just the occasional drop it might be water seeping through the dirty joint and mostly evaporating leaving salt residues.
IMO every salt water aquarist should have Aqua Knead It on hand for all sorts of jobs.