This account popped up today (@dochalladay32) and it appears like it could be the real Roy Halladay. An earlier Tweet includes a link to a fundraiser where Halladay is the featured speaker. Often, players joining Twitter (especially older ones) do so as part of some sort of charity or promotion.
Trying to find out for sure. Stay tuned.
[If it is Doc, love the choice of Chooch in the avatar.]
welcome to twitter one of the greats @Dochalladay32
— Jon Heyman (@JonHeymanCBS) March 11, 2014
Can confirm that it’s him.