To summarize:
Test kit(s) unknown
FC 3.0
CC X.X?
pH 7.4
TA XXX?
CH 550
CYA 100?
Salt 5000
Scale occurring at/around waterfall
Wondering why salt would rise that high, water tastes salty.
Did you do the diluted test on the CYA?
Salt residues will wash off easily with just a washcloth. If the scale around the waterfall is hard, try chipping off a piece, and pour a drop or two of muriatic acid on it. See if it fizzes.
People start to taste saltiness at varying levels, anywhere from 2000 ppm and up, but most people start to notice it a bit higher than that, more like 3000 or 3500.
As far as why you have high salt, there's only a few possibilities, and probably a combination of some or all of the reasons:
High evaporation (how long does the waterfall typically run per day?)
Refill with high TDS water, very little or no splashout (What's the CH and TA of your fill water?)
Much higher than normal chlorinating liquid additions (let us know your usual chlorinating liquid and approx how much you consume per week or month)
Cumulative addition of salt from some other source (midnight rambler with bags of salt??)
Testing error on salt test (I mention this because you'd pretty much need 12" of rain or a water exchange to shift salt from 6400 to 5000)