It is hard to tell without seeing it, but if there is not a huge amount of algae and the fish are doing well, maybe better to leave them where they are. Mine are outdoors, with lots of algae. I change a bit of water every day in the summer to remove some of the green algae in the water. Some green and brown algae grow on the sides of the tub, but these keep the water clean, and the fish also have fun eating them. There are suction marks where the fish have eaten the algae.
Putting the pleco in the tank for a while may solve the problem, if the temperature is over 20c and the pleco is not disturbing the goldfish.
It is hard to get a goldfish tank to work properly. They produce a lot of waste and often the water quality is very poor, causing disease problems. This is partly because they are big and active, and partly because unlike many other fish, they chew their food. This releases food particles and nutrients into the water, feeding algae or bacteria. The algae eat the nutrients and keep the water reasonably clean.
That is why I leave some algae. Although they don't look great, they keep the water clean.
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