So we know the didact once looked different to what he looks like in halo 4.
In fact we see in the halo 4 terminals the changed appearance he acquires from tampering with the composer trying to become immune to the flood.
He goes from this
(note: no fangs, he has a combat skin under suit, green/blue eyes)
to this
(note: fangs, no combat skin undersuit, his armour just seems to be part of him, red eyes)
I always wondered what the didact means by saying “the composer will not work on my new form”. I was like what new form? did you evolve? i just assumed in the terminals he had no armour on.
NOW, I believe when the librarian referred to bridging the realm of digital and organic they became abominations. I see now she meant that the didact became on of those abominations. of sorts he has become a semi organic semi digital being.
Now for my theory, when john released the didact into the composers in escalation, and the didact seemingly becomes an essence of orange composed glow. He CAN NOT become a knight in any form as he has been altered already, but he can however stay as digital essence which can later be reversed, but at a cost this will further make him an abomination and alter his appearance even more so. The composers were destroyed, but there is one on the ring that the monitor took away for repair still.
A watcher can revive a knights essence back to a full knight if its quick enough to react, so suppose jul m’dama or perhaps some thing else (unsure here) gathers the didacts essence in attempt to reverse the process, but once reversed we have a didact that just looks like an ugly mutated angry thing.
Feel free to expand on this, as i believe (or along these lines roughly) is why the didact can’t be composed but can be that digital data stuff.
I leave you with this
Through out the whole video up until 10:42 the didact is seen with regular armour and a cape, and normal looking skin, then at 10:47, he asks “did it work? run the simulation” and his appearance has totally changed as he has fallen to his knees like he has just had the life taken out of him. 

