What brush is safe to use on this grill?

Im VERY new and weve been using this grill for a bit, personal stuff happend and we couldnt clean the grill and now when we trued theres these tough bits. I try to use what i presume is a steel/metal brush, and in some places theres rust. My brother, whos not that experienced either, says the rust is because i brushed to hard and long because i removed the "non stick" on it. And that metal/steel brushes arent good for grills. Does anyone have tips?