a hitchhiker or simple propagation?

I have these little things growing on the leaves of my java fern. They started out as little root clusters, really firmly attached to the leaves, and now they are growing new leaves of their own. I dont know if this is a hitchhiker plant thats come from somewhere else in the tank and decided to leach off the java fern, or whether it is java fern 'babies' although I thought they propagated by using runners which this is not.

Anyone know what they are?