Is this Dino's?

[Werbo]

I have been battling this reoccuring brown string algae for over 3 months now. I have syphoned the sand bed and blown off the rocks numerous times only for it to return within days. 10 days ago I syphoned out the sand bed and took pictures April 26 and tonight. Stuff is on rocks and glass as well.
Other characteristics:
- Snails become lethargic, are constantly falling off the rocks and eventually slowly die
- little air bubbles sometimes develop
- no nitrates or phosphates according to salifert test kits
- alk 9 dKH, cal 420ppm and mg 1400 ppm
- tank is 13 months old
- 3 days of complete darkness did very little.

Or is this just really bad diatoms?
Any ideas?