That's probably just the dust seal (you would hope
). Grab hold of them with some needle nosed pliers and pull them out. Then push the pistons back into the caliper. You will be able to ride the bike, but this should be considered a short term fix. Get new seals fitted a.s.a.p. otherwise you will find that all the road crap and brake dust will foul the pistons.
Just a point: You do this at your own risk. Brakes are safety critical. You shouldn't really muck about with them.