It looks as if that's a tree trunk of some sort in the picture - if it is, then Jeyes fluid won't have done it any good at all. If the trunk belongs to a plant you want to retain, handling this problem depends how big the plant and its root system is - is it a tree? Is it healthy?

This is liverwort, and it grows in damp, undernourished, uncultivated and compacted soil conditions. You can use Scotts Weedol Fast Acting on it first of all, then to avoid its coming back you need to carry out remedial work. If the trunk doesn't belong to something you want to keep, dig the area over, removing any residual growth, add plenty of humus rich material such as composted animal manure. Liming will help because this stuff doesn't really like neutral soil conditions, but liming may not be appropriate for anything else growing there. Adding more planting will help so there's not just bare ground. When you've finished, add an organic mulch (bark chips or composted manure). This will degrade over time and enrich the soil as it does so, which will decrease the risk of a recurrence because liverwort doesn't like fertile, free draining soil. Plant the area so there's much less exposed soil. This answer assumes that the picture is showing a soil bed.

You might try making a 10 to 20 percent white vinegar solution and spraying that over the area to kill it. Let it stay on the area for an hour then rinse it off.

I thought it was a clothes post not a tree?

Might be, hard to say, we can only see the bottom part, could be anything.