Strange algae (?) in my sump

[Dash]

Just to follow up, I decided to keep the rock & observe this algae's growth. It spread quite rapidly, from about a dozen heads to 30 or so in this 3-week period. They are attached quite firmly & are difficult to remove so I can see them being a real nightmare in someone’s DT. Quite a few big mature ones which are filled with gross white goo. I picked the rock as clean as I could & returned to sump - interested in seeing if they will return.