No need to refrigerate, o2 will have the greatest effect on longevity of freeze dried food nutrient content. How much to feed, thats up to your fish feed enough at each feeding that they eat almost all in 5 minutes a few times a day.
The amount of time your rock has been in salt water and overall coral mass for will have the largest impact on if algae forms. Skimmers, scrubbers, refugiums, and other filtration covers the difference, this part is my opinion lol.