Help! New to the hobby!

Hey guys! Brand new to SW hobby. 4 days ago I bought a used 28 gallon nanocube from a guy who had it in his office. He completely let this thing go and I have never owned a saltwater tank before and I felt like I knew more than he did. The model of the tank is a mt-602-led. I know the nanocubes come with all differnt stock lighting. But I do believe the stock lights on this tank is a little better than average stock biocube stock lights but correct me if im wrong. My first question is are these lights going to be okay for most corals or do I need to upgrade them before proceeding. Everything is stocked besides a added nanocube protein skimmer that I will more than likely upgrade later down the road. There's 50lbs of live rock with a ton of coralline all over the tank. I would like to add a fuge in the back in the next couple of days which leads me to my next question. The guy at the LFS said I can have a fuge free floating in the second chamber is this true? Or should I take out something out of media basket and add it to it. I have a heater skimmer and media basket in second chamber already. Another nightmare I am dealing with is this tank is over stocked and it's a war zone. I'm planning on taking some fish to LFS tomorrow to get some money to put towards a fuge light. The only thing I have added is a arrow crab for the sole purpose of killing a foot and half (maybe bigger haven't seen all of him) fire worm. But the stupid arrow crab is more concerned about killing all of my CUC than the fire worm. He was tearing a part one of my emerald grabs today while the fire worm was killing snails. Yes I know fire worms are not supposed to kill livestock but I have a video of him eating a live snail straight out of his shell. But I'm going to return the arrow crab tomorrow and make a diy trap for this fireworm hopefully I will catch that on video also. My last question is I got the tank with a stupid amount of green hair algae. After water changes and renewing filter media it is now turning red and is really easy to peel off. Does this mean it's dying? When its dead do I pick it out of the tank or does it just go away? Also I have only tested phosphate and nitrate so far. Phosphate reading 0 and nitrate at 5. I have to get more test kits for other readings. My plan for the tank is to have 2 clowns and maybe 2 skunk shrimp with a ton of corals. Hopefully I can put up pics in a couple days but it looks really bad I think the 3 corals that he had in the tank died during transport but leaving them in for a couple more days to see if they progress. Let me know thanks guys. Sorry for such a long post