After trying to read the vast majority of this thread I've go my own theories.
The barnacles sound like ships only deeper in tone.
The sounds they give off are like they are very large structures with something in them, moving, living.
The harmonics may need more analysis as the sounds resemble whale calls.
One pilot mentioned that shooting the spikes sounded like the barnacle was screaming.
The fact the area the barnacles have been found in is sandy means the area is soft, easy to move, grow in and get out of when finished growing.
Something tells me this is a large living organism that is growing in the planet using the high metal content. Hence the metal alloy.
The damage to ships and SRVs suggests its feeding on the metal, possibly something to do with the sparkles that are being seen.
Possibly the same thing that is growing the structures.
Also what are the crystal structures seen on the spikes. Different type of seed perhaps.
This is first contact with a possible Alien species that is seeding/colonising this area of space and the barnacles are the result of the UA making landfall.
There may be a full size one in the bubble of the UAs hiding from us.
I would recommend against shooting them as this is more than likely hurting them and possibly leading us into war.
Remember Frontier said our actions with first contact decides how our relationship with the Aliens will progress, peace or war.
This could also be,
A search for life project by an alien race. The symbol does look distinctly like Barnards Loop Nebula. Orion's belt is part of this.
A space creature that is growing and preparing to hatch.
A possible beach head for invasion. Grow your own base.
I'd recommend close monitoring to see what is developing, but be prepared for a quick response.