In quasar.conf.js make sure you have entered QTd

framework: {
    components: [
        'QTable',
        'QTd'
    ]
}

and try change tyour code in

<q-td slot="body-cell-address" slot-scope="props" :props="props">
    <span>Custom Text: {{props.value}}</span>
</q-td>

Great! thanks!