Any idea?

Certain plants, like Java fern, reproduce by growing baby plants right out of their leaves. Now that looks like an anubias in the front left maybe? The baby plant and roots could have detached from the mother plant and the roots are just stuck in the anubias? Wave your hand around by it and see if it dislodges easily.
 
its definitely attached and i believe you are correct on the plant type. I have pulled some off the other plants and thrown in the baby tank to let them grow.
 
Yes, it is baby java fern.
I've often left them free floating to grow out a bit. If you can wedge it into a cranny on a decoration, it will anchor as it grows - (try using a toothpick).
 
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