After heading into the lower levels and the slums of Omega you will eventually end up in Mordin's hospital where he will mention his missing assistant and asks you to look for him while curing the area of the plague.
Walkthrough Editbatarians. How the encounter ends is up to Shepard and your decisions. Using Charm or Intimidate will convince the batarians that Daniel is trying to help cure the plague, causing a peaceful solution, or if Shepard chooses the batarians can be killed after letting Daniel go free. Alternatively you can signal one of your team members to shoot the lead batarian, causing Daniel's death. If he lives he will return to the clinic and leaves a medi-gel crate behind.
If Daniel lives he will send Shepard an email explaining the situation and how it has improved at the Clinic on Omega.
Note: There is a glitch (found on the PC version) that might cause one of the batarians to freeze in his place and become invulnerable. Reloading from a prior save is the best solution if this persists.