Use 'scummy' to describe surfaces or water that look visibly dirty or have a layer of grime. The bathtub is scummy and needs a good scrub.
Pro Tip 2/3
Physical Texture
'Scummy' often implies a slimy, sticky, or slippery feel, like algae on water. The rocks near the scummy edge of the lake were too slippery to walk on.
Pro Tip 3/3
Negative Connotation
Using 'scummy' suggests a strong negative reaction, indicating disgust or revulsion. She refused to drink from the scummy glass.