salt washes off just like it would wash off your hands.
it is extremely hard to tell from the photos of what is
actually going on..... just start washing with a degreaser
and brush - progress to more aggressive measures until
you understand what is going on.
Do a vinegar wash in a test spot to see if the spots go away.
If they do, then it is surface corrosion which can be dealt with.
Many members here just don't like bed liner on their hulls.
Matter of choice - your boat - your call
marine growth such as clams, oysters, barnacles, etc mostly
just die and drop off after coming out of the salt water.
If they stick, they are easily scraped off with a blade of some sort.