I'm glad to see you found him.
For starters, 5 Gallons is REALLY small... According to saltwaterfish.com
(click on the underlined), this fish needs a minimum of thirty gallons. I would not add any fish to this tank, and would upgrade as soon as posible.
The more successful hobbyists will tell you that a filter is unnecassary, and I agree. I use a protein skimmer, Liver Rock (about 1.5 lbs per gallon of water), and Live Sand (4-6" Deep). A whisper filter will collect debris that will be broken down into nitrates, and cause problems in the aquarium. You will have to clean the filter pads in the filter at least one or two times a week.
The sponge (please post a pic to help identify) probably did not survive any type of transfer into your aquarium. Sponges cannot come into direct contact with the air; they must remain submerged in water. I had one that was a hitchhiker on a rock of zoas, and it just spread over the zoas, suffocating them. I had to cut it off and try to salvage the few polyps that made it.
The small algae bloom sounds like diatoms. These are a natural process in the marine aquarium. Is it just on the glass, or on the rocks also? On the glass it can just be wiped off; on the rocks leave it be. Please post a pic of this bloom also to help us identify it.
Do you have any snails or crabs in the tank?
And did your son buy a test kit for this aquarium? Water quality is very important, and regular testing is crucial. I would like to know the parameters of your water. Test for nitrates, nitrites, ammonia, ph, salinity and temperature, and post the results. The more information we are given, the better we can help.
Welcome to the forum!