You can use VLC and do pretty much anything, the documentation is all over the place.
ffmpeg is good too, and if it's already MJPEG... the "protocol" is pretty simple with Wireshark and "Follow TCP Stream". With a MJPEG stream you could directly embed it with the html <img> tag or use JS to cause a refresh and stop hangups, hell one can do anything.
You could also use v4l2loopback and write to a /dev/videoN device with any number of video transcoders (vlc, ffmpeg) to use the mjpg streamer that grblweb is using.
If that's all nonsense to you, you could just open another tab with the IP cam's stream.
Andrew