I'd imagine the "slimy" feeling is due to all the starch passing over and through it. But how come it's so hard to clean? Even with repeated scrubbing the slickness can stubbornly remain.
I'm Italian, we use colanders to drain pasta every day. It's the starch. You are supposed to drain pasta in the kitchen sink, right? As soon as you drain the pasta, pour the pasta on the sauce, or serve it and run cold water immediately over the empty but still hot colander. It will remove the starch without effort.