Algae growth on the back glass?

[GoblinFish]

Hello everyone, i went away for 3 weeks and left my tank to my father which only fed the fish and added RO/DI water. Now that i got back home i found allot of hair algae on the back wall and also my xenia is not looking so good, all the tip of the tentacles looked burned and pale.
I also found allot of air bubbles in the sand, this could be because the skimmer was releasing a good amount of micro bubbles in the display.


I know that i am not giving much information here but i was just wondering if anyone had an idea on what's going on here.

[coraljunky]

Please post water parameters. Sounds like nitrates are present.