Hi knight and welcome. If money is tight I would get rid of the marine as, not only are inhabitants costly, they are dearer to run with the equipment and extra lighting needed. We have a trade section in here where you could sell the rock you already have.
Whitespot is usually a result of poor water quality. This may or may not be due to overcrowding. Without knowing actual tank size (dimensions rather than litres) and how many fish you have it is hard to tell. How often do you do water changes?