[Bartman]

I appear to have what I can only describe as rust coloured coralline algae spreading in my tank. I has started, very specifically, on my only 2 branches of Tonga rock. At the tips of the rock it grows out like a cap and where the rock touches the glass it is spreading onto the glass. It covers 10-15% of the branches now. The tank and rock went up in May. Any idea what it is?