You has consider if your FormArray is a FormArray of FormControls or a FormArray of FormGroups, but the problem is how you iterate over the controls,

A simple example of both

export function customValidateArray(): ValidatorFn {
    return (formArray:FormArray):{[key: string]: any} | null=>{
      let valid:boolean=true;
      formArray.controls.forEach((x:FormControl)=>{
          valid=valid && x.value=="a"
      })
      return valid?null:{error:'Not all a'}
    }
  };

export function customValidateArrayGroup(): ValidatorFn {
    return (formArray:FormArray):{[key: string]: any} | null=>{
      let valid:boolean=true;
      formArray.controls.forEach((x:FormGroup)=>{
          valid=valid && x.value.name=="a" 
      })
      return valid?null:{error:'Not all name are a'}
    }
  };

You can see an example in stackblitz

NOTE: To create the form, I use the constructor of FormGroup, FormControl and FormArray, but you can use FormBuilder too.

NOTE 2: it's not necesary enclose a FormArray in a FormGroup

You have considered if your FormArray is a FormArray of FormControls or a FormArray of FormGroups, but the problem is how you iterate over the controls,

A simple example of both

export function customValidateArray(): ValidatorFn {
    return (formArray:FormArray):{[key: string]: any} | null=>{
      let valid:boolean=true;
      formArray.controls.forEach((x:FormControl)=>{
          valid=valid && x.value=="a"
      })
      return valid?null:{error:'Not all a'}
    }
  };

export function customValidateArrayGroup(): ValidatorFn {
    return (formArray:FormArray):{[key: string]: any} | null=>{
      let valid:boolean=true;
      formArray.controls.forEach((x:FormGroup)=>{
          valid=valid && x.value.name=="a" 
      })
      return valid?null:{error:'Not all name are a'}
    }
  };

You can see an example in stackblitz

NOTE: To create the form, I use the constructor of FormGroup, FormControl and FormArray, but you can use FormBuilder too.

NOTE 2: it's not necesary enclose a FormArray in a FormGroup