Some new invasive thing...?
 

Today I noticed some oddballs growing in the tank, that seem to suddenly have appeared from nowhere. I swear they weren’t there last week. They’re growing on the underside of a plating Monti and seem to have quickly established a little “colony”.
Does anyone have an idea what they might be? They are fairly bright yellow, little hemispheres about 1/2” in diameter with a central tube. Sponges? Should I get rid of them somehow? Or are they beneficial? The tank has been running for years and I haven’t introduced anything new for a looooong time, so they just sort of magically appeared from the soup of possibilities that a reef tank is.

They are sponges I believe
Harmless and also a sign of a good tank !

Looks like common yellow sponges to me. I have a bunch of yellow sponge growth in my reef as well in shaded areas.

I had white versions of these little sponges. They spread around the shaded areas of the tank, so they will stay on the underside of your coral. Totally harmless. Mine came from an LFS that had some real old live rock brewing. Cute little buggers with their little siphons.

Tippers used to have dozens in my tank before I had my cyano invasion a year or so ago

Ughhh... Cyano takeover is the worst. I remember I had that in my very first 75g reef, definitely don’t miss that.