Limnobium laevigatum (amazon frogbit) im guessing
This looks great! The growth is very lush.
a nice set of pics there mate, one thing a do recommend you do is get some anubias nana petite in there around the rocks, with them being slow growing theyll attract algae, your ottos will last 5 mins if theyre not fed properly, and there diet should revolve around green algae, substitutes alone wont keep them alive.
I just chuck in a wee slice of cucumber or a couple of peas for my ottos - keeps them nice and fat
lovely tank, can you keep any more fish in there with the ottos or will the shrimp and 3 ottos be the limit for that tank