That's part of the Boss event (check the icon on the lower right of your screen with a dragon head symbol that says "BOSS" on it).
Click the Boss event icon and accept the quest, you'll have to kill 30 of those majins.
After you kill one of them, they might or might not drop a Silver coin. Keep saving them up.
After level 50 or 51, they'll change to being fat majins, instead.
If you hover your mouse arrow over the event icon, you'll see a % number. When it reaches 100%, huge and overpowered dragon bosses will spawn all over the Mushroom Rock maps. People usually gather to kill it at north of the South Mushroom Rocks map (just outside the south entrance of Mushroom Village), on channel 2.
After the boss dies, a robot NPC spawns in its place and you can deliver the Boss event quest and claim your reward. You can choose either EXP Bonuses or Gold coins (recommended).
Now, what can you do with these silver and gold coins? At the same robot NPC, you can buy a title for your character that gives you stats. You can get an anti-crit title for 250 silver coins, or trade 20 silver coins for 1 gold coin. Then you can also buy 2 other titles, costing 30 coins each, but these ones change every 2 months (right now we have March and April titles, but next month it'll be April and May titles instead).
The stats for these titles vary, depending on which month title you want. There's a guide that lists which stats are given for each month, here in the forums.
http://www.dbocom.com/forum/showthread.php?t=34138
Finally, you can also buy a mystery capsule that will give you random items, such as: party revives, full self-revives, and if you're really really lucky, a cooldown title. Each capsule costs 3 gold coins.
Hope that helped.