Great work on the picture!!! Now that pooldv identified it for you, you can search for efflorescence using the search box towards the upper right off the page and read the stores of similar pool owners. You may want to then add other keywords you find, for example, efflorescence muriatic acid, or, efflorescence pressure washer, or efflorescence grout. Once you have a basic grasp on things, you might have questions, which you can ask here or, if the other thread is recent/active there too.
Pressure washers and muriatic acid are both capable of doing more harm than good if used incorrectly, not to mention dangerous. They might also be your solution if used safely and correctly! Whatever you decide, probably best to try it on a spot first, just in case.