As others have mentioned, its snail eggs... I dont mind them, since they will either be food for an inhabitant, or if they survive and hatch, free CUC... only thing to watch out for is if they hatch and they are small, some can get into pumps and other moving parts and crunch up and mess them up (not usually too big of a problem), so if you hear odd grinding or crunching from your parts, clean them out.