Goldfish tanks should be species only. They will try and eat anything they can fit in their mouth and have been known to get stuck or impaled on plecos. If it's a small pleco like a dwarf bristlenose they could potentially both be killed if the pleco gets stuck.
Goldfish also produce a ton of waste which can be very problematic for more sensitive fish with the nitrate swings.
I've seen goldfish tanks where they have absolutely lovely algae carpets with crystal clear water, you could try cultivating your algae depending on what kind you have.
If you have brown algae it's typically too much food/waste debris, blue green is bacteria, and bright green can be too much light.