My mollies have had the same thing since day one, and there is nothing wrong with them that I can see. My one platy can get a string that is 3 or 4 inches long some days. Probably has something to do with diet. They get Wardley flakes and brine shrimp, but I think it is the flakes that do it because they have done it since before I started on the shrimp. I'll try Hikari flakes or a different brand when I run out of the Wardley.
With nothing else wrong then it is most likely diet. I don't see any reason to worry about it. If they stop eating, become inactive and anti-social then it might be more that the diet.
When my two gourami that died had white, stringy poop they had parasites. It can be an indicator for parasites, so you may want to keep a close eye for belly swelling or other abnormal signs. Just another point of view from experience.