I have to say I laughed when I read this one! For me it's so simple, but I could understand where the confusion comes from as the code "FP" isn't explained very well.
"FP" is a feature of the combination systems (chlorinator and filter pump together in one) that stands for "Filter Pump". It allows you to continue using the filter pump section even after your chlorinator dies on you completely (don't you love that Intex knows it will break before the pump will, hence why they have this feature). To toggle this ON or OFF, simply press and hold both the up and down arrows on the control panel at the same time until you hear a single beep from the system and the code "FP" will go away and go back to telling you the amount of hours left in the cycle.
Someone obviously had pressed those two buttons by accident and activated the filter pump only mode (which your salt water generator shouldn't have been working while this was turned on)
Hope this solves your problem!
EDIT: I just realized your model number indicates you have a standalone SWG that's NOT combined with a filter pump. That's very strange that you have a "FP" code on that system, it sounds like Intex is using the same exact board to operate both models! In any case, the only way to get that code to go away on Intex systems is by holding those two arrow buttons I mentioned above at the same time, if that doesn't work then you have me beat.