aha... Nope not magnesite. Looks just like a thin sand cement with som red pigment in. Perhaps they were trying to mimic a nice quarry tile floor. It looks a bit rubbish though.
Personally think it dangerous. It's thin, bonded so may get damp. It's also failing and if it's drum my it will fail further. I would recommend removal and reinstatement with an equivalent depth of suitable insulation (marmox etc) and tile that instead