The two code parts are not really compatible. The pulse sensor depends on a interrupt handler (a quite heavy one) doing almost the whole work. This must be called once a ms to let it work properly. The LED strip library sends the color values out using some assembler code. To get the timing more or less correct it disables interrupts during the complete transfer.
There is some code and a comment that gives me the impression that you should add the following code to your setup:
ledStrip.interruptFriendly = true;
Maybe that helps, otherwise you have to use either another LED strip or other code for your heart monitor.