Can I store premixed marine water

I'm taking a trip. I wanted to leave some water with the marine salts
already added to make it easier to change water. This is for those who
will be looking after the tank while I'm away.

Will it start fouling up... with algaes. I thought maybe storing it in
the basement. Sitting on the basement floor would keep its temperature
down to near 60 degrees if not cooler.

thank you

There should be no problem at all. Keep it in a cool place with the lid on
(or dark location). You can also put a small power head in there to
circulate the water and air-ate it. Just have the person doing the water
change stir up the water a little, heat it up to the tank temperature then
add it in there.