Rot or mold on the underside of deck

Anyone know what this stuff is?

Had boards similar looking to these on our deck two months back, ripped the bad boards out, but the neighboring boards acquired the same ailment since then. Ripped them out too (and anything with any sign of white stuff) and attempting to order copper-green wood treatment to paint underside — would this be enough to stop this from happening? Does anyone know what this stuff is? Was told that it’s mold and to just clean it off, but the affected boards broke very easily so I’m worried that it’s some sort of nasty rot. Can mold weaken boards to this extent? Does it even matter what this is or is it just a question of getting rid of anything affected and reducing moisture in the area?

This is a fixer upper house and the deck has been very neglected for a number of years. The deck lacked any sort of stain/seal and the gaps between the boards were so small they would disappear completely when it got wet. We also had a VERY wet spring and summer. Only about two weeks ago we cut the gaps. Couple of days ago stained it and redirected the gutters away from the area. The entire deck is massive (probably 800 sq ft) and the remaining boards seem fine, so would really not want to rip the whole thing out. Would this in addition to copper-green treatment on the underside be enough?