Just looks like lack of maintenance and extra nutrients. The algae has nothing else to compete with for growth, so it is taking over. This is my own opinion and what I have done with really algae overgrown picos and my 40g. Happens any time I go away on vacation.
Take the rock out in a small bucket of tank water and go to town on it with a toothbrush. Rinse really well in cleaner water. scrape all the sides down and remove the pump and magnet (I think that was the rectangular thing on wall) and clean them in regular tap water. Scrub well!
Make sure you have good RO/DI saltwater ready and do a full water change. I mean 90%. Do not worry about bacteria as it is in the rock you scrubbed clean. You do not have a large bioload with only the fish, shrimp and snail from what I see. Also, make sure the salinity (specific gravity) is the same as the shrimp could have issues if it varies too much. Also make sure temp is same on new water.
Then, suck out the tank. The side scrapings should have settled and you can vacuum sand if want. On first clean I would just do the sides and 90% change.
After new water is in, add back the rock (that you had in clean saltwater and scrubbed. Then the pumps and anything else.
Now, with a small tank like this, you do need to keep up on maintenance. Especially if you are adding anything and the algae is the main consumer. What is in there now really does not need fed much at all. Do water changes every 5 days of about 10% and watch what you feed. Should get better fast.
Again, this I have done on many of my picos and even my 40g. Yes full water changes. It helps a ton!