If you call startCameraCapture when the camera is already running, this call will simply be ignored. So this should not cause any memory problems, as long as you don't initiate new _camera instances every time. (You can check the implementation of startCameraCapture in GPUImageVideoCamera.m.)
If you are seeing memory problems, it's most likely from some other places. You may post more code here, or use instruments to track down the issue.