If water or an active hydrogen impurity (e.g. alcohol) was there, the problem would have started before refluxing, together with an exothermic reaction.
I have read somewhere, that Grignards mainly form dimers in Et2O , while mainly monomers in THF. There might be a relationship.
However, pure MeMgI in THF or Et2O solutions, has a grey and sometimes a cloudy appearance because it is rather a suspension than a solution. So, add more THF and warm it carefully (Take care that meanwhile, there is no absorbed humidity).