What is this algae on my dads glass?

[Xephash]

He has had it a while i remove it and still comes back. Any ideas what it it and how to fix? Thanks

 

[Xephash]

Here is the picture

 

[FishGuest5123]

Wow! You got me on this one. Anyone else?

 

[nic1]

Can you describe it please, it's just a bit difficult to id from this picture..... Colour, texture, how easy it comes off ect. Thanks