Is this black algae and is it related to these symptoms?

Hi, all --

Below is a photo from today of my tank (see details in my signature). The last few weeks, this blackish algae that I have circled has spread very quickly. During that time, I was away and asked a friend to feed the fish, which they did. The light, however, was left off.

Also in that time, two of my fish have died (a zebra danio and a black skirt tetra). Another two have developed growths around their mouths; a zebra has a whitish ring while a black skirt (shown in the photo -- hard to see) has a black and red growth on its 'chin'.

I gather from reading other posts that the algae is black beard algae. Is that what it looks like?

Also, does this seem linked to the deaths/growths I've seen?

Finally, will this solve the problem?:
- transfer fish to a holding tank
- toss all my ornaments/gravel
- scrub 20g tank
- reset tank with new ornaments/gravel
- replace fish with about 5 gallons from original tank

Thanks so much for your input!

I found an article elsewhere about this algae, but am still not sure if this is what I have. I disposed of some of my ornaments already and will do a bleach dip for the others, as described in the article. I think I'll just toss my only living plant -- it's covered with this algae. Then I'm going to do a 50-60% water change. Hopefully this will work! :hang: