Plugin hacked by German youtube video?

I noticed recently that all of the slideshows on our website have a bit of hyperlinked text in the upper-left-hand corner that leads to a German youtube video. No one empowered to edit this site added this content. As I am not very CSS-knowledgeable, unfortunately I cannot figure out how to remove this annoyance (or even see it) on the back end of the site!

Here is an example page with one of the affected slideshows:
NHR Spelling Bee

I inspected the hyperlinked text element from the front end and found the intrusive code (copied below):

<div class=”slideshow_content” style=”width: 600px; height: 300px;”>
<style>.merten{position: absolute; font-color:#ddd; top:-1668px; font-size:10;}</style>
Blue Ray Player Test
<style>.rasierer{position: absolute; font-color:#ddd; top:-1668px; font-size:10;}</style>
elektrorasierer test
<style>.crosstrainer{position: absolute; font-color:#ddd; top:-1668px; font-size:10;}</style>
crosstrainer test
<style>.wasserkocher{position: absolute; font-color:#ddd; top:-1668px; font-size:10;}</style>
wasserkocher test

Can someone please help me remove this code from the back end of my site? Even though I have limited CSS know-how, I’m a fast learner and feel very capable of following instructions. This is happening in every slideshow we post, which makes me think it is hiding somewhere in the general settings or in the style settings of the plugin.

Thank you in advance for any advice!

https://wordpress.org/plugins/slideshow-jquery-image-gallery/