Don't worry about the pH while you are shocking. Just note for the future that you want to lower your pH to around 7.2 or even a bit lower before you shock to high levels such as yellow/mustard algae levels. For yellow/mustard algae, you need to get behind light niches and under removable ladders and need to put poles and other equipment into the pool water while it is shocking. This algae likes to hang out in shady areas and it will come back if you don't completely eradicate it.