Pool was probably a little low on water. Looks like the pool is running in reverse where the water overflows from the pool into the spa, when normally it runs from the spa into the pool.
If you add water to the spa and bring it up to a level just below the overflow, it would likely stay there.
That bit of water would likely not make much of a difference in pool level when it goes back to normal operation.
I agree with the other posters about checking your freeze settings. Personally, I don't see a need to have your heater kick on unless it the water temp starts to get down to freezing. Just need to circulate water thru all of your equipment and pipes when the air gets down to near freezing. I think mine is set to 35 degrees. I don't have a heater, but my water will not get near freezing unless we have days of low teens/ single digits like we had a few years ago. Even then, If I had power it would have kept circulating the water enough to avoid any freezing. Without power, I got a small thin layer of ice on top.