Curious - what is this?

[buttersheltie]

New hobbyist here.
I set up a nano tank about 4 months ago with the initial plan of having a macroalgae-only display tank (along with mangroves).
Planted the mangroves a while back but, unfortunately, I managed to kill all five of them within a very short period.
After that experience, I decide to hit the pause button since most likely, with my ineptness, I will kill all the macroalgae too.
Since then, I have left the tank empty - but I do enjoy looking at the tank everyday with the pretty white sand, purple rocks, and gentle waves.

However, I notice this today. Do you know what this is?
I am surprised that something manages to grow in the tank since I have not added any nutrients to the tank at all (except from occasional moth carcass that fell into the water) and I often neglect the water poorly (either under-top-off or over-top-off).

 

[Nemo&Friends]

The first time I tried macro algae, I killed them too, especially red grapes which I thought were so pretty, they were completely gone.
I then tried other type of algae. Then one day, I saw that one red plant was growing on my rock, it was very tiny, and sure enough it was red grapes which must have seeded somehow before dying. It is not very big yet, but is is growing slowly and doing well.
If you used live rocks, it could be anything, but if you did not, it is possible than the root of your mangrove survived below the sand and are now acclimated enough to grow.
I do not know enough to identify what you have, I am just suggesting base on my experience.

 

[FUNGI]

almost looks like very young bryopsis.....just IMO

 

[buttersheltie]

After a few more weeks, my D did a bit more research and it looks like they are Acetabularia caliculus.
This is from Wiki:
Acetabularia caliculus, the umbrella alga, is a species of green alga found in shallow temperate and tropical seas. It usually grows on pebbles, shells or pieces of rock, and is often found in seagrass meadows, on mudflats and coral reefs, in estuaries and growing on the submerged roots of mangroves. Each individual thallus consists of a single cell with a long stipe and a terminal cup-shaped or flattened disc.

This makes sense - they hitch-hiked on the magroves that I bought (and killed )