There are 2 possibilities.
1.Coins or tokens are not listed on the coinmarketcap but have a special market, you can see on their telegram and you can find information on the telegram.
2.Scam, in this case the coins / tokens that you have can experience ico failure or other reasons.
I myself have a lot of coins that are number two lots of ico that fail then not continue to make coins or tokens that are not continued to buy and sell that causes the coin to only be trash in my wallet.
The trash coin can't be erased but will remain in the wallet and I think if you want your wallet to be clean, you have to make a new wallet and transfer your valuable coins or tokens.