What is this on my shell and how can I fix it?

[txg]

Pretty sure I may have ruined my shell a bit with this so I stopped before I did anymore damage. We've had our hot tub for about a month and a half and yesterday I noticed white / dull spots in a couple areas of the tub above the waterline. The guy at the hot tub store said it was ok to use a non-abrasive sponge to clean things so I went to work figuring I just hadn't been keeping up in those areas.

Most of the stuff was coming off but there were some really stubborn areas that with a fairly significant amount of pressure the shine was coming back in those spots as well. But then I noticed the white spot and I have a feeling I scrubbed went right through some protective layers or something to the bare acrylic below 

Does anyone have an idea as to what these dull spots are and how I might fix them without further hurting the tub? I have a picture linked below:

https://imgur.com/a/GWcGXCG

[A.O.]

Looks to me like the finish is gone... not sure how to fix that.

[Tman122]

Pretty sure I may have ruined my shell a bit with this so I stopped before I did anymore damage. We've had our hot tub for about a month and a half and yesterday I noticed white / dull spots in a couple areas of the tub above the waterline. The guy at the hot tub store said it was ok to use a non-abrasive sponge to clean things so I went to work figuring I just hadn't been keeping up in those areas.

Most of the stuff was coming off but there were some really stubborn areas that with a fairly significant amount of pressure the shine was coming back in those spots as well. But then I noticed the white spot and I have a feeling I scrubbed went right through some protective layers or something to the bare acrylic below 

Does anyone have an idea as to what these dull spots are and how I might fix them without further hurting the tub? I have a picture linked below:

https://imgur.com/a/GWcGXCG

Some brands have better finishes on them than others.

[txg]

Wow, that's no good. I've sent the pics to my hot tub store and they're reaching out to the manufacturer on my behalf. Thanks for the input guys!