Place a fog machine above the lasers
That is my idea, you stole my idea 
Really powerful lasers will cut steel so you don't want to put your fingers in those. You can pick an appropriate class of laser for your application though Laser safety - Wikipedia.
There have been real accidents. I am sure I recall people being blinded at a disco or some such event by a laser.
Even with a low power laser I would avoid looking directly at it and also avoid rings and things that might reflect the beam. The main risk would probably be children sticking their heads in and peering at where the "strings" come from.
Surely harp strings have to be plucked? Does where the string is plucked, the plucking technique and the strength of the pluck not all matter? How does a laser harp cater for this?