you could have two problems
one holes may be birds after bugs like leatherjacks, the

slim may be slim mould grey off white, yellow or orange

wash off and it should dissolve ther is not chemical control

the mould is unlikely to cause much damage wash off the

slime with water. Kev

Depends what you mean by fairly deep. Badgers can dig holes for their toilets and they are several inches deep and the same across. Holes made by birds are very small - the diameter of the beak.

Thank you very much for the responses. The holes are very circular and less than half an inch wide and about two/three inches deep. The grass is very mossy

I'm coming round to birds. We've got lots in our lawn that just go down to soil level but not much deeper and I'm pretty sure that's what they re.