Problem with tank....in trouble

[Drag Racer]

Came home from an extended weekend away to find my tank no where near when I left it. Everything was doing great, as it has been for the last year except for a small hair algea issue. When I came home the entire tank was covered in this brownish yellow algea and all the corals were retracted. I did a test on the water and everything checks out in the ok colume.
Ph 8.2
Am less than 20ppm
Nitrate 0
nitrite 0
phosphat 0

The only thing i dont have a kit for is Alkalinity. Could this have changed and caused the weird change? All the fish seem to be doing fine, eating and acting normal. Tank is a 55 gal with a 13 gal sump.

2 clowns
1 regal
1 scopas tank
1 lawnmower
1 pep srimp
and some crabs and snales

Any advise would be greatly appreciated.

One last note the skimmer I have is the worst you can get and way under rated for the tank, but has been used sinse the tank started up almost 2 years ago.