"mengelap" means "wiping" with the intention of cleaning or wiping something off. Example: wiping the window, wipe a table.
"mengusap" can means "petting" or the action of rubbing something slowly (though there's another word for petting in malay which is "membelai")
Example: - Dia usap lutut perlahan-lahan untuk kurangkan sakit. (She rub her knees slowly to ease the pain) - Dia sibuk usap kucing dari tadi sampai tertidur. (He's busy petting the cat since earlier that he falls asleep)