I'm not sure exactly what it is, but what I can tell is thqt its definitely grown in its emersed form. Most aquarium plants grow either on land or in the water and have different growth forms for each. Just let it be and it should eventually grow some differently shaped leaves as well as a thinner stem.
When it gets a few inches of new growth it will be safe to trim off some of the old stem and replant it.
If you want an example of emersed vs submersed growth look up pictures of water wisteria. It has vastly different leaf shape in both forms.
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor