The win moves Haladas up to fourth position - which would qualify them for the Europa League - four points off first placed Vasas Budapest.
Williams, who is this season averaging a goal every 103 minutes, scored the winner when he headed home from a long throw in.
The 28 year-old has been in inspired form in his first full season since joining the Hungarian side in January this year.
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Previously the former Melbourne City attacker scored against MTK Budapest and Diosgyori VTK.