They can go a day with out food and a lot of people do this so the octo is out a lot more on that day. 2 days might be pushing it but if you leave live food in the tank then that should be fine for a couple days.
Roy has noted that a hungry (not starving) octo may have a longer life span and suggests feeding every other day. I can't make myself do this so we fast them (and all of our tanks) one day a week (sometimes they beg a small crab on fast day - can't help it). Leaving live food in the tank seems to work very well for multi-day absences but we have not tried this approach simply because my son is usually home when we leave and I have a built in care taker.
There are a couple of places to find shore shrimp but I will recommend Paul Sachs because of my personnal experience with his consistency of availability and timely shipping (and a few favors when I needed something special over the years I have purchased from him ).
Fiddlers are air breathers but they somehow seem to do very well in an reef tank, especially if they can easily climb to a surface area (like the overflow) on occassion. I don't recall one dying naturally in my tanks (ie not eaten) even after several weeks where I do get losses in a more natural environment for them (but I keep a quantity in their holding tanks and only put a couple in the octo tanks at a time).
I am not sure what the goo is. Normally you will find a fully cleaned out shell, separated from the empty but recognizable body and empty claw(for a male crab - I break off one tip so it can't pinch). They don't come apart for awhile when they die naturally but your scavengers will start eating them soon after death.
got it out of the tank asap.I also rubbber banber a small piece of silver side to a stone and it too was covered in a semitranslucent goo.??? Ocot is alive and hiding in a rock ccrevice, see a couple arms probe out every so often.
Humm, the goo makes it interesting. It would not be from a mercatoris but it does sound like you have something that may produce slime (some octos do) in the tank. Beldar was some form of Macropus (there are a number of species) and I noticed a slime on her feeding stick as well as on uneaten but munched shrimp. I also lost a thorny star in that tank and wondered if the slime made the food poisonous (there may be no relationship, just second guessing since I have never before or since lost a thorny). It was clear though and more like thick spit. (I took a photo of it on the feeding stick but you really have to want to see it). Do you have any idea on how long the arms are compared to the mantle?