it's just "good lichen" and won't hurt. it looks like a yaupon holly (Florida native), perhaps the weeping variety?
however, i would have pruned one of those large trunks a long time ago before it grew into the other - they rub against each other in windy weather, and could cause problems since it's like an open sore where they rub.
Lichen can simply be cleaned off with water, Murphy's oil soap, and a scouring sponge.It will turn slimy green but just wash off with a hose,do a little more scrubbing and it will look so good.
The "Garden Rebel" is right about a technique for getting rid of the lichen. As Cathy says, the lichen won't hurt the plant but if it a "showpiece" on your property you may want to clean it up a bit. It will take time and and there might be other gardening chores that are more important to get done in the time you have available. In our area winter storms with ice and wind take care of some of those problems but I don't think that will occur in Melbourne.
see this Walter Reeves answer for a close up of lichens and compare it to what you have on your Holly
http://www.walterreeves.com/landscaping/lich... ...»
It does look like though you may be sustaining some injury from mowing or weed eating too close by the look of how close the grass is to the trunk
however, i would have pruned one of those large trunks a long time ago before it grew into the other - they rub against each other in windy weather, and could cause problems since it's like an open sore where they rub.