Cactus/Cacti advice
Hi all, I have had this Cactus/Cacti for around 5 years, always in the house in a sunny position. During the hot weather I put it outside. A pink flower has appeared in the top. Sometimes the flower pops out, other times it stays as it is in the photo. Will the flower always be there now or will it die off. Also any care recommendations whilst it's flowering. Thanks in advance. 
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Firstly , my congratulations on being able to preserve your cactus for five-years !
The species you have is a fine specimen of Melocactus curvispinus ssp.loboguerreroi !
(Now there's a mouthful)!
This species originates from dryish but humid woodland conditions in Western S.America , mainly Colombia .
Melocacti are regarded by cactus enthusiasts as notoriously tricky to grow , as they cannot tolerate cold at all !
The crown (growing point ) of your plant is developing what is known as a cephalium ; this is a zone of modified hair and bristles from which the insignificant tiny flowers appear ; what you see 'popping-out' are the ovoid pink fruits (which are edible) .
The main body of the plant will now stop growing , but the cephalium will continue to mature and form a cylinder of red hair above the 'cactus proper' , for want of a better term .
Treat as you do normally ; obviously your growing regimen works for you ; one important point though ; all cacti respond well to neutral or slightly acidic water .
If you've access to clean rainwater , that would be excellent . Good luck !
PS Google Melocactus for some amazing images !
used to work in the 1980's at a specialist Cactus & Succulent nursery .
Picked up the odd snippet of info whilst there (!)
Glad to have helped .
You may find this interesting ; S.American cacti in habitat !
Click on A & M at the top , then English at the bottom .
Amazing images of these marvellous plants in the 'wild' so to speak ; Enjoy !