Help to ID this please

[RandomHero]

Can anyone ID this for me please? It is 10mm across and has steadily grown from about 3mm when I got the rock 6 weeks ago. Hasn't moved, stayed in the same spot, just grew.

It's the thing that looks like a deformed hand.

 

[bonjorno2]

Flat worm?

 

[RandomHero]

Flat worm?

Not a flat worm, hasn't moved and has 5 'branches'.

 

[bonjorno2]

Idk, seems interesting for sure. Maybe you are lucky and it's some sort of sponge.