Bleaching is the loss of photosynthetic algae from the coral tissue causing the coral to become very pale in colour.
I believe your toadstool is going to shed some mucus and to help aid it do so, as already mention, place it in a higher flow area but you may want to leave him be as he may also just be sulking. Toadstools are very much like angry teens and will often just go in a mood and close up for a few days for no apparent reason and then open up again, sometimes in a good mood and go crazy with growth.
What ever your toadstool is doing, be assured you are in no danger of loosing him as they are very resilient and will go weeks closed up without any major damage.
Good luck!