Problem with discosoma

[TeoB]

Hello, can anybody tell me what's going on with my discosomas?

I am keeping them in my tank since months without any problem. They always grew fine and wednesday were like this:

http://s2.postimg.org/h30yqbdph/WP_20150325_003.jpg (http://postimg.org/image/h30yqbdph/)

However since saturday they are like this:

http://s11.postimg.org/vh80mvpa7/WP_20150329_001.jpg (http://postimg.org/image/vh80mvpa7/)

Thing is: I have several colonies and they all look the same, quite contracted. All other corals (soft and LPS) are ok. No recent technique changes to the tank. I just added a quadricolor 10 days ago, might it be disturbing the discosomas by releasing some compounds?

Otherwise:
50 liters
60w led light
total of 3000 l/h of flow

Values are very low: NO3 1mg/l and PO4 0mg/l

Thanks in advance for your help.

[organism]

Have you checked your alkalinity? Mushrooms can start tanking pretty quickly if it's too low or unstable, they usually look like that then start detaching from the rocks.