tough stain on the pool waterfall

[cornbr34d]

my parents' pool got a really tough stain issue on the vertical face of their waterfall, and they're stumped.

They've already hit the wall with full strength muriatic acid and scrubbed it with a steel brush, but it's not really budging.
We know there are chemicals to prevent this stuff in the future, but we're looking for suggestions on how to deal with the existing buildup.

Any ideas on what might finally get these stains out? Thanks for any help!

 

[Smoothsailer]

Since a steel brush did nothing, then it's probably calcium silicate scale buildup, which can be a pain. The best course of action will depend on the surface, so before I start mentioning things I'll ask, what surface are you planning on getting this off?