You must convert it to string:
char buffer[120];
const char b2h[] = {'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'}; // Look-up table
for(uint8_t i=0; i<40; i++){
buffer[i*3] = b2h[Rx_data[i]>>4]; //Convert each char to hex string
buffer[(i*3)+1] = b2h[Rx_data[i] & 0xF];
buffer[(i*3)+2] = ' '; // Add a space between chars
} // i.e, 0x33 outputs "33 "
buffer[119] = 0; // null termination
u8g2_DrawStr(&u8g2, 3, 29, buffer);