It is caused by battery acid on poorly maintained cars and particularly a growth on the positive battery terminal which is close to that area. It is important to keep battery terminals clean. It is generally recommended to lightly coat them in vaseline.
I'm not sure I'd recommend that course of action while the battery is fitted in a Minor because there are rivet holes directly beneath the battery through which the water (and acid) can seep and spoil the parcel shelf below - witness all the damp/rotted out parcel shelfs.
I do agree that boiling water will get rid of it although the acidic growth is nasty stuff and you don't want to be inhaling the fumes given off .
I have used boiling water successfully and the simple answer is to remove the battery from the car first and do it outside. When its all clean and dry, clean up the battery clamps and vasoline over the battery poles as suggested by 'ManyMinors'...........
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