As far as fish go, I'm not sure about the effects of high alkalinity on them. I'd guess they wouldn't like it since it's not normal for saltwater to be that high. But if it's bad for their health... I have no clue.
Regarding coralline algae... that high of alk
probably would cause issues, indirectly. Have you measured your Ca
levels? As you mentioned, I'd guess they're pretty low because of your high alk
. And without proper levels of Ca
(greater than 360 or so) you aren't going to have much growth, if any. When I first started my tank, my coralline growth outpaced the Ca
replenishment from my water changes and when my Ca
dropped below about 340, my coralline algae started turning white.
I don't think adding calcium will help. With that high of alkalinity, you'd most likely precipitate it out immediately and cause a little snow storm in your tank.
How'd the alk
get that high? Weekly water changes will eventually bring those levels back down to where they should be.