They usually are not an issue. They are harmless filter feeders. I've had them in the past, but have not seen any in a long time now.
Are we sure they are pineapple sponges and have they been bothering anything in the system? Just curious.
I'm not sure on this, but maybe someone will chime in, but I think they need silicate to form. I have heard of folks with sub-optimal RODI source water being the source of silicates. Just a thought.