Stupid Question Concerning Overflow Plumbing

[Madreefer]

In my overflow I have an adjustable telescopice pipe with an elbow at the top with an strainer. The height of it determines the water level in the tank. I don't know what it is called so that is the description. Question, how to take it out to clean it? If I remove the whole peice will all the water start draining in to the sump? How would I prevent this? I would like to take it out as there is some huge aptasia that are attached to it. I'm sure that is the breeding area for them. Any suggestions are greatly appreciated!