Using NiceHash miner is simple. The application switches between algorithms to maximize your profits. Using MSI Afterburner for overclocking your gpu there are a few things to consider. One algorithm may run on different settings vs another. With that being said, do not clock your gpu cards to one algorithm. Once NiceHash switches the algorithm you may find your cards crashing. Crashing cards is lost profits. Set your overclock settings at a reasonable amount so that you can mine all of the algorithms. If you are unsure about overclocking, try setting your core clock at 150 and memory clock to 350.
You can play with the settings to your specific rig. If the gpu crashes on a certain algorithm then adjust your cards to that algorithm. Once you get the highest settings for the last algorithm that gives you problems, then it should run all of them without a problem. It is better to have your rig running stable all the time than switching back and forth making nothing.
If you are still having problems you can always disable the algorithms that are giving you problems by selecting settings, then algorithm and select each card you want to remove the algorithm from by un-checking the check box. Hope this helps you out and happy mining.