1) Any fish that would be found in the mud, perhaps a word for certain flat fish. They were being imported from the Low Countries.
1471-2 14 last’ allecis albi ... 6 C modffysshe, Hull. It occurs earlier as a by-name: 1409 Et Johanni Mudfysch pro ... tegularum, Beverley. The OED notes the word in Essex and Newcastle in the sixteenth century.