It's not uncommon to see preview text that contains a string of what looks like nonsense after the text set in the template builder.
This happens because email clients do not limit themselves to your chosen pre-header text, and will reach into an email to find additional preview content. Most commonly this results in image alt text being shown as preview text.
Luckily, adjusting your image alt text is very easy.
In the drag and drop builder, select your image and set the alt text in the Content Properties menu:
In an HTML template, use the "alt" property to add alt text to an image.