The formation of Algae on seed starting mix is a common problem.
Trying to keep the peat moist in a bit of sunshine with usually limited airflow simply creates Algae growth.
The good thing is that this is not a big problem as your seeds should not sit for too long in this mix before you transplant the seeds to bigger containers or into the ground.
Take a chopstick, pencil or some other tool and skim off the Algae or just mix things up a little bit to break the suface open and allow some air to get to everything.