noun
eraser dust; eraser shavings; eraser crumbs
Refers to the small bits of rubber or debris left after using an eraser.
机の上の消しカスを手で払った。
I brushed the eraser dust off the desk with my hand.
消しカスが床に落ちている。
There are eraser shavings on the floor.
消しゴム is the eraser itself, while 消しカス refers specifically to the debris produced by using it.
Compound of 消し (keshi, 'erasing', from 消す) and カス (kasu, 'dregs, refuse'). The katakana spelling カス is conventional for this meaning.