Gypsum plaster ( eg bonding, finish, hardwall) is water soluble so I would leave a wet cloth over it for while, til it gets soft, then scrub it off as per joe above
Don't uses any 'acid type' cement removers or you may affect the grout
for gypsum I would try and avoid chemicals when water will work. Large area use a wet dust sheet, old towel etc.
You'll need a few attempts at it, but I'm afraid it's the unavoidable result of not protecting areas first, accidental spillages, or not cleaning properly when fresh.
What bonfire do you follow - not those ruffians from the Cliffe, I hope East Hoathly are much more civilised , and I liked the pub @ Fletching , back in the day
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