It looks like very wet cecatropes. Not uncommon in senior bunnies, especially when the weather changes. If you’re concerned, offer extra hay and offer water with electrolytes (pedialyte or bounce back). As a precaution, check their bottoms to make sure their vents (rectums) are clear and dry. If the bottoms are dirty also give probiotics.
I'm not really concerned, since both rabbits are acting fine, eating and drinking normally. I'll keep an eye on them, particularly the older guy, but otherwise I won't panic....I did check their bottoms earlier this evening and both were clean. However, I did see more normal cecatropes on the tube again...Not watery. I wonder if it's Lion, my senior. Could be because of weather change. Last night it suddenly dropped into the 40s and it's been chilly all day.
Like I said, my senior rabbit did have a problem with too many pellets a while ago, until I decreased the pellet intake. I guess I'll have to wait and see if it continues...