Are you talking about those dark yellowish things? If so I've been keeping tanks since the 70's and have never seen anything like that. They are way too big for snail eggs. Have you introduced anything new before they started to grow? Any new plants or something from a local stream/pond/lake? Honestly not knowing what they are I would remove the decoration, carefully remove them, squirt the decoration with some Hydrogen Peroxide 3% and leave it sit out to completely dry before putting it back in the tank. Also don't rinse the Peroxide off.