There are fade marks everywhere and the leaves have started to curl inwards. Not only that, it looks like a completely different plant to what it was when i first got it which had this grey silvery colour to it and now, it's all green. I only got this just a couple of months back. What's happening? I have it in a sunny location and water it only when the soil drys out. Also, It's placed next to a different agave and that one is doing fine. What could possibly be stressing this plant out? Is it the sun? Rain? Or perhaps the cold temperatures at night?
If it is winter where you are you can expect a change in colour. This is due to the change in light levels. The other damage is probably frost damage. I believe this is Agave desmettiana, Smooth Agave, the only Agave I know that this smooth in texture with none to very small spines along the edges. This Agave is not frost tolerant. It come from a warm region in south east Mexico. The plant is often listed as USDA Zone 8 or Zone 9, but that only means it will probably survive those conditions. It does not mean that the plant will not be damaged under those conditions.
I would recommend protecting it from frost, it might even get damage prior to frost conditions. If you decided to put it in a greenhouse, beware of dew sitting on the leaves. This will cause additional leaf damage and fungal problems. This plants would be best is kept in conditions no less than 5°C (40°F). You can bring it inside. Only water it when it is bone dry. Give it the brightest window you can offer. This would be the easiest solution. Agave are easy to keep inside for winter, if you mind those two things.
My final recommendation is to remove those damaged leaves.