Yes, that is beerstone.
No, it will not come off by scrubbing it, even with the green sponge.
Yes, it will take an acid based cleaner to get it off. BKF works for me.
No, it still will not come off easy with BKF.
It is really attached and begins to form layer after layer, get what you can off with your fingernail, then depending on how long(how many layers have formed) you can start to remove them with BKF. It may take several applications.
You can wipe it with a wet towel and it appears to be gone until it dries and voila -still there.
Feeling for any "rough" surface on your stainless will locate this stuff for you. It is like chiseling away at stone.