Some new invasive thing...?

[Phormium]

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.