My drop on google ranking could be caused by this site?

Hi guys,
starting from January my main site started to drop some position on my main KW.

I thought the drop of my rating was done by my forum. In the last days of December I made a forum and linked site and forum by each other.

Anyway, today I made Lite membership to ahrefs... And I found a backlink I never saw before.
First time seen by ahrefs: 13 Jan '16.
Just when my site started to drop position.
I tried to open that domain, but it's flagged as bad site by firefox and chrome.
Anyway, the backlink doesn't come from that site directly. Backlink comes from a subdomains that doesn't have the flag. Also, the link on subdomains are no - follow.

So, how should I view this subdomain? As a bad backlink or not?
Keep in mind my main site just have 9 domain refferal so far, and this backlink is one of them.

Is that link a 301 redirect? If the answer is yes it can be the case because penalties pass through 301 redirects. If the answer is no a single bad backlink can't affect you rankings!