Leg. Forst.

Story, first published: Infinity Science Fiction, April 1958

A ... strange gift is received by an old recluse stamp collector ...
The old man receives an alien "stamp" that turns into a yellow "goop," an alien intelligence with a passion for orderliness and a zest for work. The goop cleans up the old man's junkyard apartment, then turns his attention to the rest of Earth. As the pores spread, the whole Earth, including the business community, is on its way to becoming honest and orderly.

Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.54

What is the other attribute of the pastoral life of middle-aged middle-class all-american person in the fifties? Right, stamp collecting. Simak could not leave this important theme out of science fiction treatment, and wrote this curious piece about an alien postal stamp alive and kicking in the most amazing directions and ways... Nothing earth-shaking, though.

Abrams, Avi: Science Fiction and Fantasy Reading Experience: Clifford D. Simak