I've had a lot of baby bangaii's go from my refugium into my sump through the overflow, and they all have survived. I've since put a screen (NOT a prefilter) on the outflow, and no more have gone over the waterfall. I think your gramma may have been dying or dead already when it was sucked in. Can you modify HD eggcrate to screen your overflow? I would recommend something like that.
Mitch
Mitch