Green’s Dictionary of Slang

mouldy grub n.

[play on SE mulligrubs]

a travelling showman; thus mouldy grubbing, performing in the open air.

[UK]Hotten Sl. Dict. 181: MOULDY-GRUBS, travelling showmen, mountebanks who perform in the open air without tent or covering. Doing this is called ‘mouldy-grubbing.’.