If it is a leak of combustion gas up the side of the injector, which it looks like, then it COULD be a big deal. If it is leaking combustion gasses it looks like black oil.
Combustion gasses aren't good for you and may get into the cab, you don't want to be breathing that stuff in.
If injectors are left to leak, carbon and gunge gets up the side of the injector body and makes them a real tw@ to remove, I have seen videos where vans have been lifted off their wheels when people have pulled on stuck injectors. You can't replace an injector seal without removing the injector.
In bad cases leaking combustion gasses can damage the head at the site of the leak and you have to get the seat re-cut.
If it is a slight leak then it should be very simple to fix, it would be short sighted to ignore it.