For my algae id's I use google.
How about taking your clams out and scrubbing them off so there is nothing for the tang to pick at? Your clams may also just need to get used to having their shells picked at. They are used to not having anything bother them so any new threat will cause them to remain in a cautious state, not opening completely. Once they get used to it, they may open back up like they used to. When I kept clams in my reef tank I had tangs that would pick at the algae on their shells and it never bothered them one bit.