I have no experience with planaria, but I'm new to the hobby and therefore reading everything I can get my hands on. In my studies I've read that they are slower than shrimp. As you said in a previous post they are swarming your shrimp food. Get a nice morsel in there on a string, wait for the feeding frenzy, then pull the piece up. The shrimp will jump off while the slowpoke planaria will still be attached.
Hope that helps some.
"There are few people who will make mistakes with fire after having once been burned. Of people who regard water lightly, many have been drowned." -Yamamoto Tsunetomo