It's very possible that this is a case of correlation and not causation. Just because these things happened to coincide with one another relatively closely in time does not mean one is the cause of the other.
A film algae that covers your corals sounds like dinoflagellates to me, I've not seen the normal garden variety film algaes cover living coral tissue before.
If it is dinos, they bloom and fade, then bloom again, and could very well just happen to do so around the time that you stopped and started calcium dosing.
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