While @madmada seems to have a point, it is somewhat contradicting to what was said earlier in the show. In season 5, while Stannis Baratheon and Shireen were at Castle Black, it is mentioned a couple of times.
GILLY (gently): What do you call it in the South, what happened to
SHIREEN: Greyscale. What do you call it north of the Wall?
GILLY: I don't know. But two of my sisters had it. They both died. How
did they cure you?
SHIREEN: I don't remember. I was a baby. Lots of people came and
tried, I think. Whatever they did, it went away. What happened to your
Game of thrones, season 5, episode 2: the House of Black and White (emphasis mine)
This seems to imply that while Maester Cressen diagnosed quickly, he didn't have a ready cure. After all, many people have to come and try above all else.
Another quote from a later episode supports this:
STANNIS: When you were an infant, (Stands up and walks around his
desk), the Dornish trailer landed on Dragonstone. His goods were junk
except for one wooden doll. He'd even sewn a dress on it in the colors
of our House. No doubt he'd heard of your birth, and assumed new
fathers were easy targets. I still remember how you smiled when I put
that doll in your cradle, (Shireen smiles), and you pressed it to your
cheek. (Looks down. Shireen’s smile drops). By the time we burnt the
doll, it was too late. (Walks to the table and adjusts the objects on
it). I was told you would die. Or worse, the grayscale would go slow.
(Turns to her). Let you grow just enough to know the world before
taking it away from you. Everyone advised me to send you to the ruins
of Valyria to live out your short life with the Stone men, before the
sickness spread to the castle. I told them all to go to hell. (Shireen
smiles). I called in every maester on this side of the world. Every
healer, every apothecary. They stopped the disease and saved your
life. Because you did not belong across the world with the bloody
Stone men. (Walks towards her). You are the Princess Shireen of House
Baratheon. And you are my daughter.
Game of thrones, season 5, episode 4: Sons of the Harpy (emphasis mine).
If Maester Cressen knew to cure it simply because he found it early in an infant, he wouldn't have needed the help of every maester on this side of the world, every healer and every apothecary, would he?
None of these quotes seem to show that they really know what actually helped. Or it is just not mentioned.
Maybe however, the distinction between Jorah's greyscale and Shireen's grayscale was that they found Shireen's way earlier, which gave them the time to actually try different things, whereas Jorah's situation is a lot less sunny.
However, I would find it weird that there is no mention whatsoever about what the actual cure was, neither in the books, nor in the series. Ebrose could have said that the cure only works during early stages of the disease, or Sam could've found a book where this was noted. I think the lack of such information would make it plausible that the maesters actually don't quite know what did it.