These bugs are the larvae of some small flying insects. Generally flea flies, fruit flies, moth midges (see Baidu encyclopedia for details of the above types). They breed in the cracks of wall tiles or floor drains due to incomplete sanitation. They do not cause serious harm to human beings. If you find any, just remove them. Or use a hard brush to the crevices, the drain to scrub, add some laundry or detergent, or salt.