To have been covered with a layer of dirt or waste.
This word can make people think of something dirty or gross.
Examples:
The pond was scummed over with green algae.
The bathtub was scummed with soap residue.
The top of the water was scummed with leaves and twigs.
The kitchen sink was scummed with old food bits.
The entire room was scummed with dust and dirt.
The pool was scummed over with a thick layer of slime.