What is this hard cream colored rock type formation at the top of my fish tank?

[Aman]

A few weeks ago I noticed a rather large (about the size of a peanut shell) white rock-like object at the top of my tank. It was growing in the area just between the top rim of my tank and the top of the water in my tank. I took off whatever it was and broke it in half and it was the same color all the way through, but seemed to be held together by a clear sort of mucus.

I just noticed now that another one of these rock-type things is forming. Does anyone have any idea what this thing is? Are they healthy? Is it a sign that something is wrong with my tank?

Any help would be much appreciated! Thanks in advance.