Polyp not opening

Here is some reading;
25 Reasons why your polyps won't expand. - Reef Central Online Community

 

Thanks for the link unclejed but i think something else is wrong coz i have polyps on 2 rocks kept adjacent to each other...polyps on these rocks seems to have died (or have they actually??!!) coz the polyps of the other rock seem to have been half opened. Is it safe to keep these in the tank once they are dead. I could see some milky stuff on them so i think polyps on one of the rocks are dead?

Rock with half opened polyps


With closed polyps (mabe dead)