Do you have an urchin in the tank? What you describe sounds like urchin detritus (likely ground up rock) but there are other critters that will grind rock as well.
We see three kinds of waste from an octopus (all float), Here is one type after eating Cyclop-eeze (accounts for the red color) and a more typical look at the clump kind combined with the stringy. The third is more of an ink-like thin white cloud (may be closer to urine equivalent).
I agree that the scallop was probably too big. When over fed, they often do not eat for a day or two. We have various ideas (not quite disagreements) on how much and how often to feed and I believe the successes with the differences are species and age dependent. Everyday, every other day and even every three days are common schedules. I tend to go by when they are hungry but do fast once a week (but will often give in to a small "snack" if the animal is dancing around on the glass just after normal supper time).
I don't have an urchin that I am aware of! That is I have never seen one out and about. But new critters are always turning up out of the woodwork... Rockwork..
The waste didn't look anything like the pictures you link to there, it doesn't float and is just a fine dark powder. Mysterious.
Orion seems to have taken to a feeding schedule of once every 3 days, however this is possibly due to the fact I tend to give him large portions so I have started giving him a bit less. Partly because I would like to see him more often and partly because I don't want uneaten food polluting his tank.
On day 19 he fed well on some prawn. I also tried putting a couple of large lego blocks in for him to investigate. He seemed to like them enough to take his food and sit next to them to eat (I will add videos later today). I tried interacting with him with my hand but I think he got a little confused and thought I was dinner - he approached me fairly quickly and latched on with one leg and I flinched, which in turn caused him to jet off. He didn't hide though, so instead of risking spooking him again I just fed him. I was expecting him to approach more slowly and was worried he would latch on fully and bite. I know he can move quick when he wants to!
This morning - day 21 - I was very surprised that he was up and about at dawn waiting for me dancing on the glass when I came down to get ready for work. Not an ideal time when I have half an hour to get dressed, eat breakfast and be ready to set off! I don't think he liked that I had turned the living room light on which I think contributed to the shortness of the exchange. I tried interacting again and he came straight up near my hand but wouldn't touch. He seemed curious but nothing more. He soon retreated beneath a rock (possibly because of the lights?) so I held some prawn close for him and he latched on to it but nibbled the stick then let go instead of taking the food. I did skewer it a bit further than I have in the past so perhaps that confused him? Or maybe he wasn't too hungry? I will make sure food isn't skewered as much from now on.
I am not too worried he didn't take it - I'm sure if he was really hungry he would have had it! Tonight/tomorrow morning he should be ready to eat. I hope he isn't bored of prawn, he loved it the last few times!
Videos to follow