This question comes up every summer without fail. These are not "red spider
mites" which are much smaller & only found on plants. The creatures you
describe are clover mites. They are entirely harmless. Someone said they are
predatory, but I doubt it. I think they are only scavengers. If you find them a
nuisance, get one of the household sprays with clover mites listed on the
label. As you noticed, even if you do nothing, they go away by themselves.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)