What kind of fallacy is it when someone states that because person A has not done everything in his or her power for social cause X, then social cause X is not a worthy cause?

The main error here is the ad hominem tu quoque . The behavior of the person supporting the social cause is not an determinate of the cause's social worth. If a person was protesting a meat factory, and they were NOT a vegetarian, then they might be a hypocrite, but that does not mean that reducing animal slaughter is not a worthwhile cause.

In this example, an analogy might be the best way to demonstrate the error in the reasoning:

So if we don't sell all of possessions and give the money to the starving children, does that mean that helping starving children is not a worthy cause?

If we change the issue, it might be more clear to the arguer since they most likely are emotionally invested in the issue about which they made the fallacy.