Apparently the LED on the side is not an LED but a viewing window for the UV light. So this is bright blue. So apparently the UV is working. Its just that no light is seen through the inlet/outlet. At the outlet the bulb is blocked by an inner part but I would have thought the inlet would emit some light. Either way apparently its working.

So I have replaced the filter material in my filter because someone else said this needs to be working too to filter out the clumps of algae that the UV produces. I think it was quite old and clogged. So maybe thats the reason.

I also drained the pond and refilled and will see if this helps.