Hello,
As Danijel said, this is Google thing so you will need to find articles about SEO and read more about this.
Here is one of articles that states that hidden content WILL BE indexed:
http://webmasters.stackexchange.com/questions/35939/is-hidden-content-display-none-indexed-by-search-engines
You may want to hire SEO expert for this matter if it’s important to you.
Regarding tags in category, below is CSS. you will need to remove CSS Danijel gave you.
.single-post .blog-info .tags {display:none;}
