Though your employer does not have the same control over customers or clients as they do over employees, the duty is the same regardless of who is doing the harassing. Your employer has a duty to keep you safe from sexual harassment.
It may not be practical or advisable to bring a sexual harassment claim against the client or customer, but you can and should report it to your employer. Your employer may be able to reassign your duties to keep you away from the customer or take other steps to protect you.
If your employer refuses to take any action at all, and allows the behavior to continue, you have the same legal rights as if the harassment was being done by a co-worker or supervisor. This means the right to bring a legal claim against the employer for failure to address the harassment.
If the situation is bad enough to make the workplace intolerable, you may be able to resign and claim constructive discharge. Finally, if your employer retaliates against you for making the complaint, you may have legal claims for those actions as well.