This can be done in different ways, I'll only share how I would do it.
So I assume it is an Endless game, with scroll-to function, and the spacing of each step remains the same. Also, I'm assuming the horizontal movable space of each step is, let's say, 800px.
For the first coin:
[On start] Create object (coin) on X: random(800) + 100 and Y: -50
For random coins after every alternate step:
[coin.Y > 300, Trigger once] Create object (coin) on X: random(800) + 100 and Y: -50
A bit of trial and error, or width/height pixel idea will be needed there.
For the dissolving part... You can create a Particle of small black square, set it to 'One-time only' Rate: 100 (or more), Random X dispersion to 800 pixels, Random Y dispersion to 20. Set the angle downwards (like a gravity effect), Then to make it dissolve [after score > 1000] Make that step spawn that pixel (from the middle of the step), and destroy the step immediately.