Gloden loaches disappearing

Hello,

I have a 120 litre tank with five shubunkins and two golden loaches. I
have two questions:

1) All the plants in the tank are getting covered in some kind of brown
substance. It looks a bit like rust and it seems to stick only to the
upper surface of the leaves. I change the water regularly, have a
filter and clean the tank and glass every week. Anyone know what it is
and whether it's a problem or not?

2) This is weird. Today, the two golden loaches both disappeared. The
tank has a lid on it, so they can't have been taken out by my cat. They
did not get sucked into the filter. All I can think is that they have
buried themselves in the gravel. Is this normal behaviour? Anyone any
guesses as to what might have happened? I've had the golden loaches for
about two weeks, and the tank for about three months.
Thanks!

Charlie