Brown 'stains' on wall and ceiling.

Hello, I noticed a few weeks ago, that there are quite a lot brown stains on my wall and ceiling.
They are very hard to see.
I actually can only see them in sunlight.
If it is night, and I have turned my light on, they seem pretty much invisible.
I was wondering if this is mold, but spraying anti-mold spray on it doesn't work.
I had problems with mold in december 2014, but since then I got a hygrometer, and managed to keep the humidity at a decent rate (It's usually between 40 to 50 % now.).
I haven't had mold since then, no mold at the windows, in corners, nowhere.
My bathroom is totally fine, no brown stains at all.
Just in my living room, the left side of the room is full of it.
I was wondering if it is something harmful, or something I can just ignore.

http://imgur.com/a/EelHd
Here are pictures of some of the stains.

Thanks in advance.