is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' code example
Example: add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message
//In the app.module.ts
import { NgModule, CUSTOM_ELEMENTS_SCHEMA } from '@angular/core';
...
@NgModule({
declarations: [ ... ],
exports: [ ... ],
imports: [ ... ],
schemas: [ CUSTOM_ELEMENTS_SCHEMA ]
})