I doubt its actually Sargassum, but rather a similar brown algae similar to a kelp. Sargassum doesn't attach to substrate like your pics indicate. Sargassum is usually free floating, sorta like Chaeto.

I thought Sargassum was a typ of brown. The floating sargssum is one kind of sagassum, but if you Google it you will find there are some that attach to substrates. Isnt the internet great.

Either way, at times it can grow pretty quickly so just prune it when needed.

I think its about time I do that.