Html tags are showing on frontend in custom module textarea admin form field?

Question

I am using Testimonial extension which has textarea field in admin form.

When I am adding content with html tag, html tags are showing on frontend.

I have tried to change textarea to editor from block file but still it is not working.

Content field code in block file is:

$generalFieldset->addField(
            'testimonial_content',
            'editor',
            [
                'label'    => __('Testimonial Content'),
                'title'    => __('Testimonial Content'),
                'class'    => 'required-entry',
                'required' => true,
                'name'     => 'testimonial_content',
                'value'    => $this->testimonial,
                'config'    => $this->_wysiwygConfig->getConfig(),
                'wysiwyg'   => true,
            ]
        );

What is the issue? can anyone suggest?