If the noise you hear is hum/buzz when connection the monitors to the mixer using XLR connections fist of all check that the outer shell of the XLR connectors is NOT connected to pin 1 (ground) of the XLR connector.
You may also need to try disconnecting the XLR male pin 1 ground connection at the connector.
If what your hearing is just “hiss” that could be just your gain structure.
If you want to use the ALT OUT outputs to feed the monitors you will need to make a TRS to XLR male adapter cable. That would be handy as you get an easy way to control the monitor levels.
As Steffen said the ALT OUT does not control any XLR outputs.