The coating on the top and sides of the walls should be selected to reflect infrared light and the coating on the floor shall absorb it.
All material reflect some infrared light. So this sentence, when interpreted in the most general sense, does not say anything. What should have written is the minimum reflectance (as a ratio) of the top and side or walls and the maximum reflectance of the floor.
Given this sentence and the common interpretation, it means the walls (top and side) reflect significantly more infrared light compared to the floor. Although no frequency is specified, you can assume it is near infrared, which goes from 880nm to 980nm.
Also, note the following sentence,
The dimensions of the maze shall be accurate to within 5% or 2cm, whichever is less.
This is very important! This means your mouse has to handle errors of the maze itself. The statement does not say whether it is 5% to each side of the specified dimension or 5% as a total. I am assuming it is a total.
This still means the height of walls can range from 4.875cm to 5.125cm. You have to assume the variance can exist in the same maze.
You should design your robot so that dimension sensitive parts are easily adjustable in the field, or use some software technique to compensate for the error factor.