I don't see anything in the pics but rock and algae.
I'd just restart the tank.
I'm generally not a big fan of this approach, but it will be the easiest solution, especially in such a small tank.
Remove whatever corals you might have to a bucket with freshly made saltwater. Ditto with fish. Using a new stiff scrub brush, clean every rock in the tank then remove them to another bucket full of water. Turn off all pumps and allow the gunk to settle, then use a python siphon or similar and clean that tank like never before. When you run low on water in the tank, add more saltwater and continue until the sand is free of funk. Fill it back up, put the rocks, fish, and coral back in and enjoy your "new" tank. Because you will not have really bothered the denitrifying bacteria that lives in/on the rocks and sand, you shouldn't have any cycle to speak of.
For a 20g tank, you are looking at 2-3 hours, tops, start to finish.
You may still need to run some GFO and carbon in reactors to play catch up with any PO4 that could leech from the rocks, but that's really not that big a deal.
Good luck, and post some pics of the "new" tank!!!