Can I cast Enchant Creature cards on Enchantment Creatures (Bestow) that are on another creature?
No, because whilst Nylea's Emissary is bestowed on another creature, it isn't itself a creature - it's just an Enchantment &mdash; Aura.

Its card type might be Enchantment Creature &mdash; Cat, but whilst it's bestowed, that's overwritten to just make it an Enchantment &mdash; Aura. That's thanks to the following two rules:


  702.102a (the expanded definition of Bestow): [snipped] "Bestow [cost]" means "You may cast this card by paying [cost] rather than its mana cost." and "If you chose to pay this spell's bestow cost, it becomes an Aura enchantment and gains enchant creature.
  
  205.1a: Some effects set an object's card type. In such cases, the new card type(s) replaces any existing card types.


The text I've bolded is something Nylea's reminder text points out, though it doesn't explicitly say how it remains an enchantment in the reminder text.

As a result, whilst Nylea's Emissary is a bestowed permanent, it's just an aura and not a creature. Note in particular the "it becomes a creature again" statement in Bestow's reminder text - that wouldn't make much sense if it was a creature to begin with.

Furthermore, Thassa's emissary says "Enchant creature". Whenever something simply says "creature", it means a permanent on the battlefield that's a creature. Nylea's Emissary isn't a creature whilst it's bestowed, so it can't be enchanted. This is flipside of the mechanic that allows a Mutavault to be enchanted with something that says Enchant creature whilst in creature mode, even if its card type doesn't include "Creature".