You can use the rules from ngIf directivie in ngClass, for example:

html:

<form>
  <div class="md-form">
    <mdb-icon fas icon="envelope" class="prefix" [ngClass]="{'icon-invalid' : input.invalid && (input.dirty || input.touched)}"></mdb-icon>
    <input mdbInput mdbValidate name="input" type="text" id="form1" class="form-control" name="test" [(ngModel)]="required" #input="ngModel" required>
    <label for="form1">Required validator</label>
    <mdb-error *ngIf="input.invalid && (input.dirty || input.touched)">Input invalid</mdb-error>
    <mdb-success *ngIf="input.valid && (input.dirty || input.touched)">Input valid</mdb-success>
  </div>
</form>

css:

.icon-invalid i {
    color: red;
}