B757-200 Captain reported while parking the aircraft started to roll back. Captain then set the brakes.

Narrative

We taxied into parking spot XX in ZZZ following Marshaller instructions. I stopped the aircraft and set the parking brake. I shut down the right engine and once we had external power shut down the left engine. I turned off the beacon and cleared the First Officer to the back to disarm the crew doors. While he was in the back disarming and opening doors I received an indication from the marshaller that I perceived to be the release brakes" hand signal. I repeated the signal back to him but due to where he was standing on the tug it was difficult to see so I repeated the "release brakes" hand signal back to him and it appeared that he repeated it back to me. I released the parking brake and the aircraft started slowly rolling backward (as it sometimes does on a sloped ramp if the chocks aren't placed tightly against the tires). It seemed as though the aircraft was rolling more than usual so I stepped on the brakes and stopped the aircraft (at the same time it reached the tether limit). I set the parking brake again at that point and left it set. Ramp eventually came back up on the intercom and told us they needed to tow us forward (approximately 2 feet); which we did with no further incident. After the stairs were mated the (first time) marshaller and his supervisor came up on the flight deck to discuss what happened. He was indicating to set the brakes each time while I was seeing him indicating to release the brake. We discussed the poor lighting of the marshaller when standing on the tug at the position they were at (I could mostly just see the marshaller's wands; hand; and part of his face in the light of the wands) and the supervisor said they are supposed to be fixing the lighting with LEDs.Maintenance and the First Officer inspected the nose gear and nose tether and observed everything looked normal. We set up and flew our ZZZ-ZZZ1 without further issues. Cause: My failure to positively and clearly coordinate release of the parking brake with the ground marshaller due to a misperception of hand signals being received in the deficient lighting in ZZZ parking spot XX. Suggestions: 1. Better lighting in parking spot XX at ZZZ.2. By my not releasing the parking brake until absolutely sure that is the signal I'm receiving. A lack of clear signals was what caused me to repeat the perceived signal back to the marshaller; and my perception that I saw the same signal again. In the future if there is any ambiguity I will keep the parking brake set until I receive voice confirmation that chocks are in and the brake can be released. 3. Clearer hand signals; aided by better lighting."