This is a lily pipe. A nice expensive easily broken way of getting the water back into your tank.
And yes that is algae across the back. That's where the most is.
That is symptomatic of poor CO2 distribution (along with too much light).
I made a spray bar out of PVC tube (22mm I think) to try with my 600l/hr Juwel internal filter, to try before converting my tank to high tech, as I didn't want to buy an external filter. It was a failure as flow was clearly not enough for my tank width. The flow did move across the top of the tank, but really nothing at front substrate level, put a bit of fish poo at front, it moved a bit and just settled down and plants at rear of tank hardly moved at all. For a planted high tech tank should be at least x10 flow for tank volume, so 1800litres/hour in my case, 600l/hr clear won't work.
I assume you have a Juwel 260, as 2 x 54W, you will be looking at least 2600l/hr filtration rate, the internal 1000l/hr falling hopelessly short, even with half the media removed as suggested and a powerhead.
Something like a JBL1501 (1400l/hr) or even the monster JBL1900 (1900l/hr) in conjunction with internal filter and power head are unfortunately the only way forward if you wish to remain high tech.
Also CO2 injection via glass diffuser on a tank this size won't cut it (as shown by your fine algae specimens).
I started on my 180litre with a JBL 1501 (1400l/hr) + 600l/hr internal filter and glass CO2 diffuser and it was pretty obvious I had CO2 distribution issue, patches of reoccurring algae on the glass. Main give away was putting the drop checker at various places around the tank it was blue/green at lights on time rather than lime green when placed optimally in the tank. An inline CO2 diffuser on filter outlet (and later a power head as still got blue green drop checker at bottom front left) got a lime green drop checker where ever I placed it in the tank.