You are a good friend indeed. To take care of a friends tank is one thing but to seek the advice of others is great
Welcome to the forum
You say the tank has finished it's cycle? If so can you post the numbers? NH3
If all's well, I would make sure you do a large water change to bring down any remaining nitrates and keep things up to par. How are the remaining fish?
As far as the algae is concerned, this is quite normal for a new tank. Depending on the water used, tap or RO
, it should subside as the tank ages a bit. Nothing to be concerned about as yet. Was any money left behind for the possible addition of a few snails(?) providing the cycle is done?