Ha!
Sorry, I forgot I'd posted this. It's macro algae, it was grey/lilac under my PCs in the little tank and went dark purple/blue after being under the t5's. Being under the stronger light seems to slow it's growth and yeah, it has the potential to grow out of hand but is easy enough to prune. In this tank it grows slowly enough, if you were closer I'd send you a handful, it's got a pretty wicked colour. i had another thread trying to ID it as while back, when it was under the PC lights, I'll go look up what it was called.
Edit: Here's the link with what it used to look like.
https://forums.saltwaterfish.com/t/300860/id-pls-violet-branching-macroalgae-picture
I *think* it's related to the red gracilaria
All the white rocks are new, as you can prolly tell
they're LR but they don't have any colour yet
That'll change soon enough. See the white specks on the back wall? 'pods lol, they're eeeverywhere