[Minsc]

This one spontaneously grew out of my aquasoil one day. The growth habit is exactly like HM, with both vertical and horizontal stems present, however there are only 2 leaves per node, and the leaves themselves are very thin.
My best guess so far is Crassula helmsii.

 

[HeyPK]

That would be my guess, too.

 

[RPB]

My guess: Microcarpaea minima.

In my tank:

 

[Minsc]

My guess: Microcarpaea minima.

Wow, never heard of that one! It very well could be.
Anyone know how to tell?