Hi Lozbug,
I'm having difficulty identifying this. The one shot from the side looks like it might be Blue Green Algae (BGA) covering the gravel near the glass. It alos looks like you may have wiped something off the glass which may have been Green Dust Algae (GDA). I'm having trouble with the pot though.
I'm looking at your tank details and nothing looks outrageous (other than the assumption that NH4 can ever be zero). A 25G with 30 watts of T8 isn't excessive by any means and the tank should have cycled in three months. It took me a while to realize that "1x Tosakin, 1x Ranchu & 4x wcmm" is referring to the type and quantity of goldfish, so the fish loading doesn't seem excessive, unless they are very large and/or are known to be messy.
As always, my first suspicion is an NH4 spike so I'd have to ask what kind of maintenance schedule you do, such as water change and filter cleaning. What kind of filter media are you using and did you clean the media too much are obvious questions. Also, did you suddenly just put the fish in recently? Could something have died and gone unnoticed? Do you overfeed and perhaps have uneaten food rotting in crevices somewhere?
The current concensus is that GDA, (if that's what it is) has to be left to it's own devices for 3-4 weeks, and then it goes away. If it were confirmed that the coating on the gravel were BGA, and if the tank were planted I would say dose more NO3 but this seems incongruous in an unplanted tank. You could try a 3 day blackout. Long term, more frequent water changes might help, but what bugs me is that it took three months for this to occur. That indicates to me something happened recently that lowered the water quality.
I'm afraid my attempts at ID are purely speculative. Check JamesC's algae guide and see if you can compare with his photos. There's not enough acuity in these photos so I'm trying to judge strictly on color and location. Sorry I couldn't be more definitive. Perhaps another member can help here?
Cheers,