Handjobs, with or without the "happy ending", are never a risk for any STD, including HIV.
- if there is some semen left or other fluids on his hand from the previous customer
No risk. Hands never transmit STDs, but by the time the last customer left to when you got there, hands would be exposed to air, movement, etc., which would all kill anything. All STDs die quickly once outside the body.
- they usually use some wet small towel to clean my penis i hope they change it but what if there is some semen left from under customer
It's clean. Even if it wasn't, there is no risk. The semen would have to get into your urethra to cause any infection from the semen.
Are there any risk from these thing? Is it safe to get happy endings ?
No risk, and yes, it's safe.