"chalky" residue on shell

[Brian VT]

I use softened water at fill up.
I use bromide+bromine tabs and bleach per the sticky here.
I use a Taylor test kit and maintain the water perfectly.
I just did a drain/refill and noticed that, below the water line, the tub looks "chalky".
What is it from and what's the best way to remove it? (it didn't come off with a sponge+Ahh-Some)
The tub shell is a textured plastic, not a high-gloss acrylic.

 

[Texas Splash]

Brian, can you post a full set of test results please? That will be very helpful as we try to evaluate your situation. Thanks.
FC
CC
CYA
pH
TA
CH

 

[Brian VT]

I don't have the exact #s handy but they are always kept at recommended levels.

 

[Texas Splash]

Typically when we hear of a chalky residue situation, we think of a potential calcium issue. But it's not always as easy as just getting a CH reading or assuming right away it's scale. Not that we don't trust your capabilities, but seeing all the results would be of great value. Just whenever you get a chance, try to post those for us so we can put everything into the poolmath calculator ourselves and compare.