If you want to attempt to keep it, feeding phytoplankton will be a key factor. I have seen several of these in people's tanks. But, I cannot say anything about the long-term success these people have had. The death of the bivalve may not have a significant effect on your tank. But, if you want to be safe, I would say the best thing for the Tubastrea would be to:
1. Break the bivalve into a one-valve (break it in half)
2. Separate the halves discarding the empty side and keeping the side with the Tubastrea
3. Rinse and scrub the inside of the shell, removing any remaining tissue.
4. Quickly place the tubastrea back into your tank. Offer it something to eat later in the night to see if it will produce a feeding response.
This will be much less stressful than attempting to break the Tubastrea off of the shell (which will die in the process anyway).