Brake fluid seeping?

If you mean it is between the hard line and the fitting on the rubber line, Tighten the line nut down farther to compress the line tighter. The seal is formed as the line compresses into the seat inside the fitting, normally you tighten them until you achieve a leak free connection. If the line won't seal then take it apart and look at the seat in the line and the flare on the hardline, if you reused the hardline it may take more to seal because it was already deformed to seal in the old fitting. More than once I've had to reform the bubble on that style fitting because it was not able to seal.

Those are not really used with a bubble flare, they are for a 45 degree double flare, and even then they are not a great idea in brake systems due to the pressures involved during stops. However it's your vehicle.

They have a lower working pressure than the parts around them. Brake systems can generate 2000 psi or more in a panic stop. Those are sold more for things like fuel lines with lower pressures. It may even say somewhere on there that they are not to be used on brake systems.

No need to replace the line. Just cut the end off and create a new bubble flare. Then tighten it back in place. Or leave the washer in place and just watch it to be sure it doesn't start leaking again, the most likely failure would be that it cracks at the edge where the two pieces seat and then starts leaking, it won't be an explosive failure.

They are normally a one time use because the initial flare is slightly oversized and when installed they get crimped into the smaller size that seals them. However you can be fortunate and just tightening it down a bit stops the leak if the parts are close in fit. As far as the flare goes, a bubble flare is much easier than a double flare. The problem is usually having room to put the flare tool like the first image in place, however there are flaring tools that you can get as a loaner tool that look like the second image that are much easier to use. You just need the tools to tighten the tool in place and rotate the anvil part to make the proper flare.

The flex lines can come apart that way, the hard lines generally fail with pinholes or they rust off at a bracket. I've replaced a lot of rotted lines over the years.

Heat or a razor blade to scrape it off, both will work. Be sure none of it is in the area you make the bubble. The flaring tool won't clamp on it very well.