Bubble algae questions.

[BuddyBonButt]

Don't have much expectations asking this question, but when it comes to bubble algae, how do they grow? To give an example to my question:

A couple days ago I found a fairly small shell covered in bubble algae. I removed it very carefully from my tank and looked around. I didn't see the algae anywhere else. Well this morning I found two more shells that I haven't seen, looked at, or touched in over 3 weeks. From a previously deceased snail. These shells had bubble algae on them but again no sign of them on the rocks. The shells could have had them for a long time and I just didn't notice, I don't have anything interacting with the shells to even consider looking at them.

So my question is, how easy is it for them to spread and grow? I have since removed the shells, I heard some people just pop them I heard some people don't pop them, I was wondering if I can get some clarification on this. I'll attach a picture of the first shell that I found. It's kind of blurry but I was trying to zoom in. No other bubble were any larger than these on the other shells. Although I'm pretty sure one of them popped because I have a fox face in the tank and it looks like he chewed on it

 

[HB AL]

My fish (triggers) ate them all. In the past my rabbit fish and naso tang would munch them all up.