Is this a colony?

[paullondon]

I can't find what this thing is... I thought is a colony of diatoms but not sure anymore... It is brown/redish and perfectly round, sometimes a withe little thing (tail like) wiggles from the center but not on all of them, it's very hard to scrape of the glass and reappears in 2-3 days. When I scraped them off I was left with that outer circle on the glass and that is really hard to clean, I had to run the magnets about 20 times over it to get it off the glass.
Anyone has any idea what this could be?

 

[Sugar Magnolia]

Coralline algae. Normal growth on glass, rocks, powerheads. You'll needs to get a Kent scraper or just use a razor blade to scrape it off.

 

[paullondon]

Yes, they are on powerheads and everywhere! Interesting aas I got purple coraline on rocks but not on glass so far... well untill now and I got puzzled up by the perfect round shape and that tail or whatever that string is in the center sometimes... but when I scraped them I was almost sure it's a type of algae... they are so hard to remove.
Good to know they come in rounds at first....