Custom validation rule in laravel

I'm trying to create a custom validation rule for laravel but I keep getting the following error: Method [validateOriginalAlt] does not exist.

I have 2 classes.

FormValidator.php

abstract class FormValidator extends Validator
{

    protected $validator;
    protected $validation;
    protected $messages = [];
    protected $rules = [];

    function __construct(Validator $validator)
    {
        $this->validator = $validator;
    }


    public function validate(array $formData)
    {
        $this->validation = $this->validator->make($formData, $this->getValidationRules(), $this->getValidationMessages());

        if ($this->validation->fails()) {
            throw new FormValidationException('Validation Failed', $this->getValidationErrors());
        }

        return true;
    }

    public function setRules($rules)
    {
        $this->rules = $rules;
        return $this;
    }

    protected function getValidationRules()
    {
        return $this->rules;
    }

    protected function getValidationMessages()
    {
        return $this->messages;
    }

    protected function getValidationErrors()
    {
        return $this->validation->errors();
    }
}

Show.php

class Show extends FormValidator
{

    protected $rules = [
        'title' => 'required|unique:shows,title',
        'slug' => 'unique:shows,slug',
        'flexget' => 'boolean',
        'airing' => 'boolean',
        'flexget_titles' => 'original_alt',
    ];


    public function validateOriginalAlt($string)
    {
        dd($string);
    }
}

In my controller I call the validation method like so:

    try {
        $this->showValidator->setRules([
            'title' => 'required',
            'slug' => 'unique:shows,slug,' . $id,
            'flexget_titles' => 'original_alt',
        ])->validate($data);
    } catch (Animekyun\Forms\FormValidationException $e) {
        return Redirect::back()->withErrors($e->getErrors())->withInput();
    }

Any ideas on what I could be doing wrong?