In Pandemic, when there are 2 or 3 cubes of the same color on a cured city, does the Containment Specialist remove only 1 cube when he lands or all of the cubes (like when treating) since it is cured?
The key verb here is treat.
From the game base rules (emphasis mine):
Remove 1 disease cube from the city you are in, placing it in the cube supply next to the board. If this disease color has been cured (see Discover a Cure below), remove all cubes of that color from the city you are in.
The Medic can remove all cubes from a cured disease simply by entering it but it's his role privilege, other roles still have to spend a second action to cure all the cubes
The Medic removes all cubes, not 1, of the same color when doing the Treat Disease action. If a disease has been cured, he automatically removes all cubes of that color from a city, simply by entering it or being there. This does not take an action.
For instance, the containment specialist drives to Atlanta where there are 3 blue cubes (1 action). By doing so, he removes a cube for free, but only one since it's not a Treat action. Then he treats the blue disease thus removing the remaining two cubes (1 action)
When you enter a city with 2 or more cubes of the same color, remove 1 cube
When you enter a city with 2 or more disease cubes of the same color, remove 1 of them.
Upon entering the city with two or more cubes of the same color, the Containment Specialist removes 1 of those cubes - that's it.
This ability has nothing to do with treating the disease.
In particular it doesn't care whether the disease is cured or not.