Numerous black patches of grass all over the lawn, with little white eggs clinging to the grass stems, see attached pictures.
No interactions present.
They (possibly black ants) may have built a small raised mound with the eggs inside it: this is friable and easily broken up by mowing or other gardening work, leaving the eggs exposed.
They do no harm, and eat some garden pests.
How about a slime mould - I cannot do ids
Perhaps a little unusual to find more than one aggregation in one spot, but I'm no expert on these enigmatic creatures.
Still not sayings its a slime mould but last year in a meadow naer me there were many tens of groups of moulds.
Lat/Lng: 51.4, -0.4
OS grid ref: TQ1470