Don't leave home without it
I understood a "slime coat" to be something specifically excreted by a fish, so unless someone took theirs off, I'd go with fungus or some sort of primitive animal as Matt suggests. Looks a lot like what I see in fry grow-out tanks where there are lots of fine uneaten but wholesome organic particulates piling up (or getting trapped in plant leaves). Probably can't know for sure unless you throw it under a microscope but even a child's one would do.
Who's that big orange fellow/dame in the background?