The eye clouds naturally whenever it is injured or "invaded" (for want of a better word for the presence of foreign bodies!). This helps the eye heal quicker, by allowing it to rest, so the pig has much less vision in that eye until it has improved.
The eye will need examining to determine if there is an injury, infection or foreign body present. It's fairly likely that a simple hay poke caused a slight injury, in which case you only need to worry about monitoring the progress of the eye as it heals. An antibiotic eye ointment - such as Optrex Infected Eyes (for humans, from pharmacies), or a veterinary prescription one such as Fucithalmic Eye Drops - can help significantly whether an infection is present or not.