Identity of a green substance formed during electrolysis reaction?
Without further details of your equipment, this might be a possible answer. I assume the electrolyte you added was table salt (NaCl).

What you saw as a yellowish cloud was gaseous chlorine which discharged on the clip you connected to the positive electrode (cathode) of your battery. The reason why you did see this, instead of seeing just colorless bubbles of oxygen, is because of something called "overpotential". It's a joint property of the species in your cell and the material and surface termination of the electrode; in your case, the clip (made of wire steel) has a lower overpotential for the discharge of chloride anions (from the salt you added) than oxygen.

Elemental chlorine is a very reactive oxidant, and you had a thin chlorine film around your clip electrode. The reaction between chlorine and iron in aqueous solution most probably yielded iron (II) chloride tetrahydrate which is the reason you saw the greenish colour of the previously submerged fraction of the clip.