yellow slime on lawn

Recently (end of October) we have found patches of a bright yellow slime on the lawn. It washes off but returns next day. It is in an area where moss is a repeated problem (shaded all winter by the house). The soil is not the best, despite at least 3 feeds a year, & the sub structure is chalk. Any ideas as to what this might be and how to treat it?
Derek Blackshaw

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