Yep, you can still use it. But, you will need to cure the rock to kill off any organics that may have grown into or on the rock during the time it sat outside in the freshwater.
Instead of wasting saltwater, you can always cure it in freshwater to clean off the organics. I say freshwater only because you aren't really concerned with cycling the rock right now, just curing/cleaning it. In a tote container place the rock with a heater and powerhead...turn up the heat to around 82-85 degrees to promote faster decay of organics. If the water smells, then drain completely and refill...hence reason to use freshwater, cheaper. Then, after a week or so, you can transfer to saltwater.
Another option is to soak it in a bleach/water mixture. This cleans off the old organics quickly as well. I haven't done this on a large scale, but there are many threads you can research on R2R that deal with exactly how to do this. Afterwards, it is best to let the rock dry out completely in the sun for a few days to remove any residual chlorine.
As previously mentioned in the messages above...vinegar/water mixture can also be used. This is usually around a 50/50 mixture so can be a little more expensive than the bleach options.
