Can an exempt employee be docked wages?

Don't confuse exempt status and salaried status: exempt is a function of your job duties and *not* being hourly (primarily; you also must earn a certain minimum amount). Salaried is how you are paid, and that is at the discretion of the employer: they can choose to make any any employee hourly or salaried. If the employee is made hourly, she loses her exempt status and instead is eligible for overtime, but the fact that someone would be exempt if they are salaried does not require them to be salaried--the employer may freely make them hourly and accept the consequences that the employee would now be eligible for overtime if/when they work more than 40 hours per week.
So your employer could choose to switch you from salaried to hourly; that is legal. You will be eligible to receive overtime, but that's a nonissue if you don't work more than 40 hours per seek. 
Also, unless you have a written employment contract locking in or guarantying your pay, you are an employee at will and your employer  may change--including reduce--your pay at will.
So based on what you write, they may do this: shift you to hourly even though you had previously been salaried exempt, and set your hourly rate at an amount equivalent to your weekly salary divided by the hours you had worked, and this is legal despite its unfortunate effect on your earnings.