Team conflict in a large research project
Deal with the issue head-on: send an email apologising for the ambiguity and saying that you weren't trying to be critical or suggest others weren't pulling their weight. That you wrote the email in haste and didn't phrase it well as a result.

Don't wait for others to draw their own conclusions. It's very easy to misunderstand a terse email.

It's probably also worth talking to the grad student in person, if you haven't already done so, to make the same points and defuse any ill feeling.