Cat,
I replaced my 400w mh's last week and not only the clam but at least three other corals reacted badly to the increased light. They need the light, but they also need to be acclimated. It's just like you getting a sunburn the first warm day in the spring, and the same exposure in late summer has no effect.
Wait a day to see if reduced water flow has an effect before making any other changes.
fwiw, imo, etc,
Agu
btw, I thought my clam was a goner, but it's opened and looks as good as ever. It's tough watching something that you think is dying and not knowing what to try.