It is fungus (no algae is white). It may be safe, or it may be toxic, the only way to tell is through examination by a microbiologist who can identify the species. Of course, if fish begin showing symptoms leading to death, the fungus is most likely toxic. But not all fungus is toxic. I cannot see the wood clearly (and even if I could, I might not know) but the toxic fungus is common on branchy wood often called spiderwood or grapewood. Azalea wood and the related rhododendron wood are highly toxic in themselves.