The asteroid's surface is darker than charcoal at optical wavelengths. Amateur astronomers who want to get a glimpse at YU55 will need a telescope with an aperture of 6 inches (15 centimeters) or larger.
Yeah, it's the low albedo that kills you. Something that large made of ice would probably be quite bright.
Quick OOM calc: This thing has an area proportional to the moon of (200m/1700km)^2. The moon has an albedo of ~0.12. An object with albedo 1 would thus have a brightness of ~1e-7 of the Moon's. The full Moon has a magnitude of -12.7, so such an object would have a magnitude of -12.7-2.5log10(1e-7), or about 4.8. That's clearly visible to the naked eye at a dark site, but not very impressive.
Quick OOM calc: This thing has an area proportional to the moon of (200m/1700km)^2. The moon has an albedo of ~0.12. An object with albedo 1 would thus have a brightness of ~1e-7 of the Moon's. The full Moon has a magnitude of -12.7, so such an object would have a magnitude of -12.7-2.5log10(1e-7), or about 4.8. That's clearly visible to the naked eye at a dark site, but not very impressive.