Cao Tzu is an NPC that our pirate meets after finally summoning Maruzame Castle and defeating all of the bosses. As was revealed in this dialogue and in Marco Pollo's letter to Merle Ambrose, Cao Tzu helped Marco Pollo create the map to El Dorado and was his teacher. From both of these sources and from an interesting post from Swordroll's Blog, it can be inferred that Marco Pollo learned how to write some of the map using the Scroll of Secrets, which Cao Tzu had in his possession at the time.
When we reach Cao Tzu in the game, he tell us that he needs all of the map pieces and "tables" before he can help us translate it. Evidently, it seems that one day we will make a return to Mooshu before embarking on a journey to El Dorado.
Ancient Mooshu Message next to Mooshu
another message written in Ancient Mooshu next to Mooshu's icon on the left edge of the map
Looking at the longer message shown above in red, it can be seen that there are 10 different characters to this alphabet, which, unlike the Crossbones Code, actually looks like it belongs to a language. A couple of the symbols look like they are flipped versions of each other, similar to how "b" looks like "p". Another thing that's interesting about the red message is that it is written vertically while the other message is horizontal. Also, only the characters in the second, longer line of the red message can be found in the second message.
Honestly, I'm kind of stumped on this one too.
We still have two options, though:
We can get a clear answer from these two messages with more analysis and deciphering skills.
We're not actually meant to solve these messages because Cao Tzu will do that for us in the game itself. Maybe we as the players cannot possibly glean any information from this because these messages can't be translated; the characters in the game will take a look at the map for us, presumably once our pirate has collected all 7 pieces, and just tells us what it says.
In Mooshu there exists what seems to be the full Ancient Mooshu alphabet, but it also remains difficult to be deciphered:
In this example of Ancient Mooshu, there are 18 different characters, but most of them are just rotations of one another. So, there are really 9 unique characters. That isn't to say only 9 letters can be deciphered, it just seems to be another example of KingsIsle copying + pasting and slighting reformatting existing content.
As of the present, the Ancient Mooshu message and overall alphabet remains unknown.